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Chapter 4. Virtual machine instances

OpenStack Compute (nova) is the central component that provides virtual machines on demand. Compute interacts with the Identity service (keystone) for authentication, the Image service (glance) for images to launch instances, and the dashboard service for the user and administrative interface.

With Red Hat OpenStack Platform (RHOSP) you can easily manage virtual machine instances in the cloud. The Compute service creates, schedules, and manages instances, and exposes this functionality to other OpenStack components. This chapter discusses these procedures along with procedures to add components like key pairs, security groups, host aggregates and flavors. The term instance in OpenStack means a virtual machine instance.

4.1. Managing instances

Before you can create an instance, you need to ensure certain other OpenStack components (for example, a network, key pair and an image or a volume as the boot source) are available for the instance.

This section discusses the procedures to add these components, create and manage an instance. Managing an instance refers to updating, and logging in to an instance, viewing how the instances are being used, resizing or deleting them.

4.1.1. Adding components

Use the following sections to create a network, key pair and upload an image or volume source. Use these components when you create an instance that is not available by default. You must also create a new security group to allow SSH access to the user.

  1. In the dashboard, select Project.
  2. Select Network > Networks, and ensure there is a private network to which you can attach the new instance (to create a network, see Creating a Network section in the Networking Guide).
  3. Select Compute > Access & Security > Key Pairs, and ensure there is a key pair (to create a key pair, see Section 4.2.1.1, “Creating a key pair”).

  4. Ensure that you have either an image or a volume that can you can use as a boot source:

  5. Select Compute > Access & Security > Security Groups, and ensure you have created a security group rule (to create a security group, see Project Security Management in the Users and Identity Management Guide).

4.1.2. Launching an instance

Launch one or more instances from the dashboard.

Note

Instances are launched by default using the Launch Instance form. However, you can also enable a Launch Instance wizard that simplifies the steps required. For more information, see Appendix B, Enabling the launch instance wizard.

  1. In the dashboard, select Project > Compute > Instances.
  2. Click Launch Instance.
  3. Complete the fields (* indicates a required field), and click Launch.

One or more instances are created and launched based on the options provided.

Caution

It is not possible to launch an instance with a Block Storage (cinder) volume if the root disk size is larger than the HDD of the Compute node. Use one of the following workarounds to allow an instance to be launched with a Block Storage volume:

  • Use a flavor with the root disk and ephemeral disk set to 0.
  • Remove DiskFilter from the NovaSchedulerDefaultFilters configuration.

4.1.2.1. Launching instance options

The following table outlines the options available when you use the Launch Instance form to launch a new instance. The same options are also available in the Launch instance wizard.

Table 4.1. Launch Instance Form options
TabFieldNotes

Project and User

Project

Select the project from the list.

 

User

Select the user from the list.

Details

Availability Zone

Zones are logical groupings of cloud resources in which you can place your instance. If you are unsure, use the default zone (for more information, see Section 4.4, “Managing host aggregates”).

 

Instance Name

A name to identify your instance.

 

Flavor

The flavor determines what resources to give the instance, for example, memory. For default flavor allocations and information about creating new flavors, see Section 4.3, “Managing flavors”.

 

Instance Count

The number of instances to create with these parameters. 1 is preselected.

 

Instance Boot Source

Depending on the item selected, new fields are displayed to select the source:

Access and Security

Key Pair

The specified key pair is injected into the instance and is used to remotely access the instance using SSH (if neither a direct login information or a static key pair is provided). Usually one key pair per project is created.

 

Security Groups

Security groups contain firewall rules which filter the type and direction of the instance network traffic. For more information about configuring groups, see Project Security Management in the Users and Identity Management Guide).

Networking

Selected Networks

You must select at least one network. Instances are typically assigned to a private network, and then later given a floating IP address to enable external access.

Post-Creation

Customization Script Source

You can provide either a set of commands or a script file, which runs after the instance is booted (for example, to set the instance host name or a user password). If Direct Input is selected, write your commands in the Script Data field; otherwise, specify your script file.

Note

Any script that starts with #cloud-config is interpreted as using the cloud-config syntax, For information about the syntax, see http://cloudinit.readthedocs.org/en/latest/topics/examples.html).

Advanced Options

Disk Partition

By default, the instance is built as a single partition and dynamically resized as needed. However, you can choose to manually configure the partitions yourself.

 

Configuration Drive

If selected, OpenStack writes metadata to a read-only configuration drive that is attached to the instance when it boots (instead of to Compute’s metadata service). After the instance has booted, you can mount this drive to view its contents and provide files to the instance.

4.1.3. Updating an instance

You can update an instance by selecting Project > Compute > Instances, and selecting an action for that instance in the Actions column. Use actions to manipulate the instance in a number of ways:

Table 4.2. Update instance options
ActionDescription

Create Snapshot

Snapshots preserve the disk state of a running instance. You can create a snapshot to migrate the instance, as well as to preserve backup copies.

Associate/Disassociate Floating IP

You must associate an instance with a floating IP (external) address before it can communicate with external networks, or be reached by external users. Because there are a limited number of external addresses in your external subnets, it is recommended that you disassociate any unused addresses.

Edit Instance

Update the instance’s name and associated security groups.

Edit Security Groups

Add and remove security groups to or from this instance using the list of available security groups (for more information on configuring groups, see Project Security Management in the Users and Identity Management Guide).

Console

View the instance console in the browser for easy access to the instance.

View Log

View the most recent section of the instance console log. When opened, you can view the full log by clicking View Full Log.

Pause/Resume Instance

Immediately pause the instance (you are not asked for confirmation); the state of the instance is stored in memory (RAM).

Suspend/Resume Instance

Immediately suspend the instance (you are not asked for confirmation); like hibernation, the state of the instance is kept on disk.

Resize Instance

Display the Resize Instance window (see Section 4.1.4, “Resizing an instance”).

Soft Reboot

Gracefully stop and restart the instance. A soft reboot attempts to gracefully shut down all processes before restarting the instance.

Hard Reboot

Stop and restart the instance. A hard reboot effectively shuts down the power of the instance and then turns it back on.

Shut Off Instance

Gracefully stop the instance.

Rebuild Instance

Use new image and disk-partition options to rebuild the image (shut down, re-image, and re-boot the instance). If encountering operating system issues, this option is easier to try than terminating the instance and starting from the beginning.

Terminate Instance

Permanently destroy the instance (you are asked for confirmation).

You can create and allocate an external IP address, see Section 4.2.3, “Creating, assigning, and releasing floating IP addresses”

4.1.4. Resizing an instance

To resize an instance (memory or CPU count), you must select a new flavor for the instance that has the right capacity. If you are increasing the size, remember to first ensure that the host has enough space.

  1. Ensure communication between hosts by setting up each host with SSH key authentication so that Compute can use SSH to move disks to other hosts. For example, Compute nodes can share the same SSH key.

  2. Enable resizing on the original host by setting the allow_resize_to_same_host parameter to True for the Controller role.

    Note

    The allow_resize_to_same_host parameter does not resize the instance on the same host. Even if the parameter equals True on all Compute nodes, the scheduler does not force the instance to resize on the same host. This is the expected behavior.

  3. In the dashboard, select Project > Compute > Instances.
  4. Click the instance’s Actions arrow, and select Resize Instance.
  5. Select a new flavor in the New Flavor field.

  6. If you want to manually partition the instance when it launches (results in a faster build time):

    1. Select Advanced Options.
    2. In the Disk Partition field, select Manual.
  7. Click Resize.

4.1.5. Connecting to an instance

You can access an instance console by using the dashboard or the command-line interface. You can also directly connect to the serial port of an instance so that you can debug even if the network connection fails.

4.1.5.1. Accessing an instance console by using the dashboard

You can connect to the instance console from the dashboard.

Procedure

  1. In the dashboard, select Compute > Instances.
  2. Click the instance’s More button and select Console. console access
  3. Log in using the image’s user name and password (for example, a CirrOS image uses cirros/cubswin:)).

4.1.5.2. Accessing an instance console by using the CLI

You can connect directly to the VNC console for an instance by entering the VNC console URL in a browser.

Procedure

  1. To display the VNC console URL for an instance, enter the following command: 

    $ openstack console url show <vm_name>
+-------+------------------------------------------------------+
| Field | Value					     	       |
+-------+------------------------------------------------------+
| type  | novnc					               |
| url	| http://172.25.250.50:6080/vnc_auto.html?token=       |
|	| 962dfd71-f047-43d3-89a5-13cb88261eb9         	       |
+-------+-------------------------------------------------------+
  2. To connect directly to the VNC console, enter the displayed URL in a browser.

4.1.6. Viewing instance usage

The following usage statistics are available:

  • Per Project

    To view instance usage per project, select Project > Compute > Overview. A usage summary is immediately displayed for all project instances.

    You can also view statistics for a specific period of time by specifying the date range and clicking Submit.

  • Per Hypervisor

    If logged in as an administrator, you can also view information for all projects. Click Admin > System and select one of the tabs. For example, the Resource Usage tab offers a way to view reports for a distinct time period. You might also click Hypervisors to view your current vCPU, memory, or disk statistics.

    Note

    The vCPU Usage value (x of y) reflects the number of total vCPUs of all virtual machines (x) and the total number of hypervisor cores (y).

4.1.7. Deleting an instance

  1. In the dashboard, select Project > Compute > Instances, and select your instance.
  2. Click Terminate Instance.
Note

Deleting an instance does not delete its attached volumes; you must do this separately (see Delete a Volume in the Storage Guide).

4.1.8. Managing multiple instances simultaneously

If you need to start multiple instances at the same time (for example, those that were down for compute or controller maintenance) you can do so easily at Project > Compute > Instances:

  1. Click the check boxes in the first column for the instances that you want to start. If you want to select all of the instances, click the check box in the first row in the table.
  2. Click More Actions above the table and select Start Instances.

Similarly, you can shut off or soft reboot multiple instances by selecting the respective actions.

4.2. Managing instance security

You can manage access to an instance by assigning it the correct security group (set of firewall rules) and key pair (enables SSH user access). Further, you can assign a floating IP address to an instance to enable external network access. The sections below outline how to create and manage key pairs, security groups, floating IP addresses and logging in to an instance using SSH. There is also a procedure for injecting an admin password in to an instance.

For information on managing security groups, see Project Security Management in the Users and Identity Management Guide.

4.2.1. Managing key pairs

Key pairs provide SSH access to the instances. Each time a key pair is generated, its certificate is downloaded to the local machine and can be distributed to users. Typically, one key pair is created for each project (and used for multiple instances).

You can also import an existing key pair into OpenStack.

4.2.1.1. Creating a key pair

  1. In the dashboard, select Project > Compute > Access & Security.
  2. On the Key Pairs tab, click Create Key Pair.
  3. Specify a name in the Key Pair Name field, and click Create Key Pair.

When the key pair is created, a key pair file is automatically downloaded through the browser. Save this file for later connections from external machines. For command-line SSH connections, you can load this file into SSH by executing: 

# ssh-add ~/.ssh/os-key.pem

4.2.1.2. Importing a key pair

  1. In the dashboard, select Project > Compute > Access & Security.
  2. On the Key Pairs tab, click Import Key Pair.
  3. Specify a name in the Key Pair Name field, and copy and paste the contents of your public key into the Public Key field.
  4. Click Import Key Pair.

4.2.1.3. Deleting a key pair

  1. In the dashboard, select Project > Compute > Access & Security.
  2. On the Key Pairs tab, click the key’s Delete Key Pair button.

4.2.2. Creating a security group

Security groups are sets of IP filter rules that can be assigned to project instances, and which define networking access to the instance. Security group are project specific; project members can edit the default rules for their security group and add new rule sets.

  1. In the dashboard, select the Project tab, and click Compute > Access & Security.
  2. On the Security Groups tab, click + Create Security Group.
  3. Provide a name and description for the group, and click Create Security Group.

For more information on managing project security, see Project Security Management in the Users and Identity Management Guide.

4.2.3. Creating, assigning, and releasing floating IP addresses

By default, an instance is given an internal IP address when it is first created. However, you can enable access through the public network by creating and assigning a floating IP address (external address). You can change an instance’s associated IP address regardless of the instance’s state.

Projects have a limited range of floating IP address that can be used (by default, the limit is 50), so you should release these addresses for reuse when they are no longer needed. Floating IP addresses can only be allocated from an existing floating IP pool, see Creating Floating IP Pools in the Networking Guide.

4.2.3.1. Allocating a floating IP to the project

  1. In the dashboard, select Project > Compute > Access & Security.
  2. On the Floating IPs tab, click Allocate IP to Project.
  3. Select a network from which to allocate the IP address in the Pool field.
  4. Click Allocate IP.

4.2.3.2. Assigning a floating IP

  1. In the dashboard, select Project > Compute > Access & Security.
  2. On the Floating IPs tab, click the address' Associate button.

  3. Select the address to be assigned in the IP address field.

    Note

    If no addresses are available, you can click the + button to create a new address.

  4. Select the instance to be associated in the Port to be Associated field. An instance can only be associated with one floating IP address.
  5. Click Associate.

4.2.3.3. Releasing a floating IP

  1. In the dashboard, select Project > Compute > Access & Security.
  2. On the Floating IPs tab, click the address' menu arrow (next to the Associate/Disassociate button).
  3. Select Release Floating IP.

4.2.4. Logging in to an instance

Prerequisites:

To first load the key pair file into SSH, and then use ssh without naming it:

  1. Change the permissions of the generated key-pair certificate. 

    $ chmod 600 os-key.pem

  2. Check whether ssh-agent is already running: 

    # ps -ef | grep ssh-agent

  3. If not already running, start it up with: 

    # eval `ssh-agent`

  4. On your local machine, load the key-pair certificate into SSH. For example: 

    $ ssh-add ~/.ssh/os-key.pem
  5. You can now SSH into the file with the user supplied by the image.

The following example command shows how to SSH into the Red Hat Enterprise Linux guest image with the user cloud-user: 

$ ssh cloud-user@192.0.2.24
Note

You can also use the certificate directly. For example: 

$ ssh -i /myDir/os-key.pem cloud-user@192.0.2.24

4.2.5. Injecting an admin password into an instance

You can inject an admin (root) password into an instance using the following procedure.

  1. In the /etc/openstack-dashboard/local_settings file, set the change_set_password parameter value to True. 

    can_set_password: True

  2. Set the inject_password parameter to "True" in your Compute environment file. 

    inject_password=true

  3. Restart the Compute service. 

    # service nova-compute restart

When you use the nova boot command to launch a new instance, the output of the command displays an adminPass parameter. You can use this password to log into the instance as the root user.

The Compute service overwrites the password value in the /etc/shadow file for the root user. This procedure can also be used to activate the root account for the KVM guest images. For more information on how to use KVM guest images, see Section 1.2.1.1, “Using a KVM guest image with Red Hat OpenStack Platform”

You can also set a custom password from the dashboard. To enable this, run the following command after you have set can_set_password parameter to true. 

# systemctl restart httpd.service

The newly added admin password fields are as follows:

dashboard

These fields can be used when you launch or rebuild an instance.

4.3. Managing flavors

Each created instance is given a flavor (resource template), which determines the instance’s size and capacity. Flavors can also specify secondary ephemeral storage, swap disk, metadata to restrict usage, or special project access (none of the default flavors have these additional attributes defined).

Table 4.3. Default Flavors
NamevCPUsRAMRoot Disk Size

m1.tiny

1

512 MB

1 GB

m1.small

1

2048 MB

20 GB

m1.medium

2

4096 MB

40 GB

m1.large

4

8192 MB

80 GB

m1.xlarge

8

16384 MB

160 GB

The majority of end users will be able to use the default flavors. However, you can create and manage specialized flavors. For example, you can:

  • Change default memory and capacity to suit the underlying hardware needs.
  • Add metadata to force a specific I/O rate for the instance or to match a host aggregate.
Note

Behavior set using image properties overrides behavior set using flavors (for more information, see Section 1.2, “Managing images”).

4.3.1. Updating configuration permissions

By default, only administrators can create flavors or view the complete flavor list (select Admin > System > Flavors). To allow all users to configure flavors, specify the following in the /etc/nova/policy.json file (nova-api server):

"compute_extension:flavormanage": "",

4.3.2. Creating a flavor

  1. As an admin user in the dashboard, select Admin > System > Flavors.

  2. Click Create Flavor, and specify the following fields:

    Table 4.4. Flavor Options
    TabFieldDescription

    Flavor Information

    Name

    Unique name.

     

    ID

    Unique ID. The default value, auto, generates a UUID4 value, but you can also manually specify an integer or UUID4 value.

     

    VCPUs

    Number of virtual CPUs.

     

    RAM (MB)

    Memory (in megabytes).

     

    Root Disk (GB)

    Ephemeral disk size (in gigabytes); to use the native image size, specify 0. This disk is not used if Instance Boot Source=Boot from Volume.

     

    Epehemeral Disk (GB)

    Secondary ephemeral disk size (in gigabytes) available to an instance. This disk is destroyed when an instance is deleted.

    The default value is 0, which implies that no ephemeral disk is created.

     

    Swap Disk (MB)

    Swap disk size (in megabytes).

    Flavor Access

    Selected Projects

    Projects which can use the flavor. If no projects are selected, all projects have access (Public=Yes).

  3. Click Create Flavor.

4.3.3. Updating general attributes

  1. As an admin user in the dashboard, select Admin > System > Flavors.
  2. Click the flavor’s Edit Flavor button.
  3. Update the values, and click Save.

4.3.4. Updating flavor metadata

In addition to editing general attributes, you can add metadata to a flavor (extra_specs), which can help fine-tune instance usage. For example, you might want to set the maximum-allowed bandwidth or disk writes.

  • Pre-defined keys determine hardware support or quotas. Pre-defined keys are limited by the hypervisor you are using (for libvirt, see Table 4.5, “Libvirt Metadata”).
  • Both pre-defined and user-defined keys can determine instance scheduling. For example, you might specify SpecialComp=True; any instance with this flavor can then only run in a host aggregate with the same key-value combination in its metadata (see Section 4.4, “Managing host aggregates”).

4.3.4.1. Viewing metadata

  1. As an admin user in the dashboard, select Admin > System > Flavors.
  2. Click the flavor’s Metadata link (Yes or No). All current values are listed on the right-hand side under Existing Metadata.

4.3.4.2. Adding metadata

You specify a flavor’s metadata using a key/value pair.

  1. As an admin user in the dashboard, select Admin > System > Flavors.
  2. Click the flavor’s Metadata link (Yes or No). All current values are listed on the right-hand side under Existing Metadata.
  3. Under Available Metadata, click on the Other field, and specify the key you want to add (see Table 4.5, “Libvirt Metadata”).
  4. Click the + button; you can now view the new key under Existing Metadata.

  5. Fill in the key’s value in its right-hand field.

    flavor metadata

  6. When finished with adding key-value pairs, click Save.
Table 4.5. Libvirt Metadata
KeyDescription

hw:action

Action that configures support limits per instance. Valid actions are:

  • cpu_max_sockets - Maximum supported CPU sockets.
  • cpu_max_cores - Maximum supported CPU cores.
  • cpu_max_threads - Maximum supported CPU threads.
  • cpu_sockets - Preferred number of CPU sockets.
  • cpu_cores - Preferred number of CPU cores.
  • cpu_threads - Preferred number of CPU threads.
  • serial_port_count - Maximum serial ports per instance.

Example: hw:cpu_max_sockets=2

hw:NUMA_def

Definition of NUMA topology for the instance. For flavors whose RAM and vCPU allocations are larger than the size of NUMA nodes in the compute hosts, defining NUMA topology enables hosts to better utilize NUMA and improve performance of the guest OS. NUMA definitions defined through the flavor override image definitions. Valid definitions are:

  • numa_nodes - Number of NUMA nodes to expose to the instance. Specify 1 to ensure image NUMA settings are overridden.
  • numa_cpus.0 - Mapping of vCPUs N-M to NUMA node 0 (comma-separated list).
  • numa_cpus.1 - Mapping of vCPUs N-M to NUMA node 1 (comma-separated list).
  • numa_mem.0 - Mapping N MB of RAM to NUMA node 0.
  • numa_mem.1 - Mapping N MB of RAM to NUMA node 1.
  • numa_cpu.N and numa_mem.N are only valid if numa_nodes is set. Additionally, they are only required if the instance’s NUMA nodes have an asymetrical allocation of CPUs and RAM (important for some NFV workloads).
Note

If the values of numa_cpu or numa_mem.N specify more than that available, an exception is raised.

Example when the instance has 8 vCPUs and 4GB RAM:

  • hw:numa_nodes=2
  • hw:numa_cpus.0=0,1,2,3,4,5
  • hw:numa_cpus.1=6,7
  • hw:numa_mem.0=3072
  • hw:numa_mem.1=1024

The scheduler looks for a host with 2 NUMA nodes with the ability to run 6 CPUs + 3072 MB, or 3 GB, of RAM on one node, and 2 CPUS + 1024 MB, or 1 GB, of RAM on another node. If a host has a single NUMA node with capability to run 8 CPUs and 4 GB of RAM, it will not be considered a valid match.

hw:watchdog_action

An instance watchdog device can be used to trigger an action if the instance somehow fails (or hangs). Valid actions are:

  • disabled - The device is not attached (default value).
  • pause - Pause the instance.
  • poweroff - Forcefully shut down the instance.
  • reset - Forcefully reset the instance.
  • none - Enable the watchdog, but do nothing if the instance fails.

Example: hw:watchdog_action=poweroff

hw:pci_numa_affinity_policy

You can use this parameter to specify the NUMA affinity policy for PCI passthrough devices and SR-IOV interfaces. Set to one of the following valid values:

  • required: The Compute service only creates an instance that requests a PCI device when at least one of the NUMA nodes of the instance has affinity with the PCI device. This option provides the best performance.
  • preferred: The Compute service attempts a best effort selection of PCI devices based on NUMA affinity. If this is not possible, then the Compute service schedules the instance on a NUMA node that has no affinity with the PCI device.

  • legacy: (Default) The Compute service creates instances that request a PCI device when either:

    • The PCI device has affinity with at least one of the NUMA nodes; or
    • The PCI devices do not provide information on their NUMA affinities.

Example: hw:pci_numa_affinity_policy=required

hw_rng:action

A random-number generator device can be added to an instance using its image properties (see hw_rng_model in the "Command-Line Interface Reference" in Red Hat OpenStack Platform documentation).

If the device has been added, valid actions are:

  • allowed - If True, the device is enabled; if False, disabled. By default, the device is disabled.
  • rate_bytes - Maximum number of bytes the instance’s kernel can read from the host to fill its entropy pool every rate_period (integer).
  • rate_period - Duration of the read period in seconds (integer).

Example: hw_rng:allowed=True.

hw_video:ram_max_mb

Maximum permitted RAM to be allowed for video devices (in MB).

Example: hw:ram_max_mb=64

quota:option

Enforcing limit for the instance. Valid options are:

  • cpu_period - Time period for enforcing cpu_quota (in microseconds). Within the specified cpu_period, each vCPU cannot consume more than cpu_quota of runtime. The value must be in range [1000, 1000000]; 0 means no value.
  • cpu_quota - Maximum allowed bandwidth (in microseconds) for the vCPU in each `cpu_period. The value must be in range [1000, 18446744073709551]. 0 means no value; a negative value means that the vCPU is not controlled. cpu_quota and cpu_period can be used to ensure that all vCPUs run at the same speed.
  • cpu_shares - Share of CPU time for the domain. The value only has meaning when weighted against other machine values in the same domain. That is, an instance with a flavor with 200 will get twice as much machine time as an instance with 100.
  • disk_read_bytes_sec - Maximum disk reads in bytes per second.
  • disk_read_iops_sec - Maximum read I/O operations per second.
  • disk_write_bytes_sec - Maximum disk writes in bytes per second.
  • disk_write_iops_sec - Maximum write I/O operations per second.
  • disk_total_bytes_sec - Maximum total throughput limit in bytes per second.
  • disk_total_iops_sec - Maximum total I/O operations per second.
  • vif_inbound_average - Desired average of incoming traffic.
  • vif_inbound_burst - Maximum amount of traffic that can be received at vif_inbound_peak speed.
  • vif_inbound_peak - Maximum rate at which incoming traffic can be received.
  • vif_outbound_average - Desired average of outgoing traffic.
  • vif_outbound_burst - Maximum amount of traffic that can be sent at vif_outbound_peak speed.
  • vif_outbound_peak - Maximum rate at which outgoing traffic can be sent.

Example: quota:vif_inbound_average=10240

In addition, the VMware driver supports the following quota options, which control upper and lower limits for CPUs, RAM, disks, and networks, as well as shares, which can be used to control relative allocation of available resources among tenants:

  • cpu_limit - Maximum CPU frequency available to a virtual machine (in MHz).
  • cpu_reservation - Guaranteed minimum amount of CPU resources available to a virtual machine (in MHz).
  • cpu_shares_level - CPU allocation level (shares) in the case of contention. Possible values are high, normal, low, and custom.
  • cpu_shares_share - The number of allocated CPU shares. Applicable when cpu_shares_level is set to custom.
  • memory_limit - Maximum amount of RAM available to a virtual machine (in MB).
  • memory_reservation - Guaranteed minimum amount of RAM available to a virtual machine (in MB).
  • memory_shares_level - RAM allocation level (shares) in the case of contention. Possible values are high, normal, low, and custom.
  • memory_shares_share - The number of allocated RAM shares. Applicable when memory_shares_level is set to custom.
  • disk_io_limit - Maximum I/O utilization by a virtual machine (in I/O operations per second).
  • disk_io_reservation - Guaranteed minimum amount of disk resources available to a virtual machine (in I/O operations per second).
  • disk_io_shares_level - I/O allocation level (shares) in the case of contention. Possible values are high, normal, low, and custom.
  • disk_io_shares_share - The number of allocated I/O shares. Applicable when disk_io_shares_level is set to custom.
  • vif_limit - Maximum network bandwidth available to a virtual network adapter (in Mbps).
  • vif_reservation - Guaranteed minimum network bandwidth available to a virtual network adapter (in Mbps).
  • vif_shares_level - Network bandwidth allocation level (shares) in the case of contention. Possible values are high, normal, low, and custom.
  • vif_shares_share - The number of allocated network bandwidth shares. Applicable when vif_shares_level is set to custom.

4.4. Managing host aggregates

A single Compute deployment can be partitioned into logical groups for performance or administrative purposes. OpenStack uses the following terms:

  • Host aggregates - A host aggregate creates logical units in a OpenStack deployment by grouping together hosts. Aggregates are assigned Compute hosts and associated metadata; a host can be in more than one host aggregate. Only administrators can see or create host aggregates.

    An aggregate’s metadata is commonly used to provide information for use with the Compute scheduler (for example, limiting specific flavors or images to a subset of hosts). Metadata specified in a host aggregate will limit the use of that host to any instance that has the same metadata specified in its flavor.

    Administrators can use host aggregates to handle load balancing, enforce physical isolation (or redundancy), group servers with common attributes, or separate out classes of hardware. When you create an aggregate, a zone name must be specified, and it is this name which is presented to the end user.

  • Availability zones - An availability zone is the end-user view of a host aggregate. An end user cannot view which hosts make up the zone, nor see the zone’s metadata; the user can only see the zone’s name.

    End users can be directed to use specific zones which have been configured with certain capabilities or within certain areas.

4.4.1. Enabling host aggregate scheduling

By default, host-aggregate metadata is not used to filter instance usage. You must update the Compute scheduler’s configuration to enable metadata usage:

  1. Open your Compute environment file.

  2. Add the following values to the NovaSchedulerDefaultFilters parameter, if they are not already present:

    • AggregateInstanceExtraSpecsFilter for host aggregate metadata.

      Note

      Scoped specifications must be used for setting flavor extra_specs when specifying both AggregateInstanceExtraSpecsFilter and ComputeCapabilitiesFilter filters as values of the same NovaSchedulerDefaultFilters parameter, otherwise the ComputeCapabilitiesFilter will fail to select a suitable host. See Table 4.7, “Scheduling Filters” for further details.

    • AvailabilityZoneFilter for availability zone host specification when launching an instance.
  3. Save the configuration file.
  4. Deploy the overcloud.

4.4.2. Viewing availability zones or host aggregates

As an admin user in the dashboard, select Admin > System > Host Aggregates. All currently defined aggregates are listed in the Host Aggregates section; all zones are in the Availability Zones section.

4.4.3. Adding a host aggregate

  1. As an admin user in the dashboard, select Admin > System > Host Aggregates. All currently defined aggregates are listed in the Host Aggregates section.
  2. Click Create Host Aggregate.
  3. Add a name for the aggregate in the Name field, and a name by which the end user should see it in the Availability Zone field.
  4. Click Manage Hosts within Aggregate.
  5. Select a host for use by clicking its + icon.
  6. Click Create Host Aggregate.

4.4.4. Updating a host aggregate

  1. As an admin user in the dashboard, select Admin > System > Host Aggregates. All currently defined aggregates are listed in the Host Aggregates section.

  2. To update the instance’s Name or Availability zone:

    • Click the aggregate’s Edit Host Aggregate button.
    • Update the Name or Availability Zone field, and click Save.

  3. To update the instance’s Assigned hosts:

    • Click the aggregate’s arrow icon under Actions.
    • Click Manage Hosts.
    • Change a host’s assignment by clicking its + or - icon.
    • When finished, click Save.

  4. To update the instance’s Metadata:

    • Click the aggregate’s arrow icon under Actions.
    • Click the Update Metadata button. All current values are listed on the right-hand side under Existing Metadata.
    • Under Available Metadata, click on the Other field, and specify the key you want to add. Use predefined keys (see Table 4.6, “Host Aggregate Metadata”) or add your own (which will only be valid if exactly the same key is set in an instance’s flavor).

    • Click the + button; you can now view the new key under Existing Metadata.

      Note

      Remove a key by clicking its - icon.

    • Click Save.

      Table 4.6. Host Aggregate Metadata
      KeyDescription

      filter_tenant_id

      If specified, the aggregate only hosts this tenant (project). Depends on the AggregateMultiTenancyIsolation filter being set for the Compute scheduler.

4.4.5. Deleting a host aggregate

  1. As an admin user in the dashboard, select Admin > System > Host Aggregates. All currently defined aggregates are listed in the Host Aggregates section.

  2. Remove all assigned hosts from the aggregate:

    1. Click the aggregate’s arrow icon under Actions.
    2. Click Manage Hosts.
    3. Remove all hosts by clicking their - icon.
    4. When finished, click Save.
  3. Click the aggregate’s arrow icon under Actions.
  4. Click Delete Host Aggregate in this and the next dialog screen.

4.5. Scheduling hosts

The Compute scheduling service determines on which host (or host aggregate), an instance will be placed. As an administrator, you can influence where the scheduler will place an instance. For example, you might want to limit scheduling to hosts in a certain group or with the right RAM.

You can configure the following components:

  • Filters - Determine the initial set of hosts on which an instance might be placed (see Section 4.5.1, “Configuring scheduling filters”).
  • Weights - When filtering is complete, the resulting set of hosts are prioritized using the weighting system. The highest weight has the highest priority (see Section 4.5.2, “Configuring scheduling weights”).
  • Scheduler service - There are a number of configuration options in the /var/lib/config-data/puppet-generated/<nova_container>/etc/nova/nova.conf file (on the scheduler host), which determine how the scheduler executes its tasks, and handles weights and filters.

In the following diagram, both host 1 and 3 are eligible after filtering. Host 1 has the highest weight and therefore has the highest priority for scheduling.

Scheduling Hosts

4.5.1. Configuring scheduling filters

You define the filters you want the scheduler to use by adding or removing filters from the NovaSchedulerDefaultFilters parameter in your Compute environment file.

The default configuration runs the following filters in the scheduler:

  • RetryFilter
  • AvailabilityZoneFilter
  • ComputeFilter
  • ComputeCapabilitiesFilter
  • ImagePropertiesFilter
  • ServerGroupAntiAffinityFilter
  • ServerGroupAffinityFilter

Some filters use information in parameters passed to the instance in:

The following table lists all the available filters.

Table 4.7. Scheduling Filters
FilterDescription

AggregateImagePropertiesIsolation

Only passes hosts in host aggregates whose metadata matches the instance’s image metadata; only valid if a host aggregate is specified for the instance. For more information, see Section 1.2.1, “Creating an image”.

AggregateInstanceExtraSpecsFilter

Metadata in the host aggregate must match the host’s flavor metadata. For more information, see Section 4.3.4, “Updating flavor metadata”.

 

This filter can only be specified in the same NovaSchedulerDefaultFilters parameter as ComputeCapabilitiesFilter when you scope your flavor extra_specs keys by prefixing them with the correct namespace:

  • ComputeCapabilitiesFilter namespace = "capabilities:"
  • AggregateInstanceExtraSpecsFilter namespace = "aggregate_instance_extra_specs:"

AggregateMultiTenancyIsolation

A host with the specified filter_tenant_id can only contain instances from that tenant (project).

Note

The tenant can still place instances on other hosts.

AllHostsFilter

Passes all available hosts (however, does not disable other filters).

AvailabilityZoneFilter

Filters using the instance’s specified availability zone.

ComputeCapabilitiesFilter

Ensures Compute metadata is read correctly. Anything before the : is read as a namespace. For example, quota:cpu_period uses quota as the namespace and cpu_period as the key.

ComputeFilter

Passes only hosts that are operational and enabled.

DifferentHostFilter

Enables an instance to build on a host that is different from one or more specified hosts. Specify different hosts using the nova boot option --different_host option.

ImagePropertiesFilter

Only passes hosts that match the instance’s image properties. For more information, see Section 1.2.1, “Creating an image”.

IsolatedHostsFilter

Passes only isolated hosts running isolated images that are specified using isolated_hosts and isolated_images (comma-separated values).

JsonFilter

Recognises and uses an instance’s custom JSON filters:

  • Valid operators are: =, <, >, in, ⇐, >=, not, or, and
  • Recognised variables are: $free_ram_mb, $free_disk_mb, $total_usable_ram_mb, $vcpus_total, $vcpus_used
 

The filter is specified as a query hint in the nova boot command. For example:

--hint query='['>=', '$free_disk_mb', 200 * 1024]'

MetricsFilter

Use this filter to limit scheduling to Compute nodes that report the metrics configured by using metrics/weight_setting.

To use this filter, add the following configuration to your Compute environment file: 

parameter_defaults:
  ComputeExtraConfig:
    nova::config::nova_config:
      DEFAULT/compute_monitors:
        value: 'cpu.virt_driver'

By default, the Compute scheduling service updates the metrics every 60 seconds. To ensure the metrics are up-to-date, you can increase the frequency at which the metrics data is refreshed using the update_resources_interval configuration option. For example, use the following configuration to refresh the metrics data every 2 seconds: 

parameter_defaults:
  ComputeExtraConfig:
    nova::config::nova_config:
      DEFAULT/update_resources_interval:
        value: '2'

NUMATopologyFilter

Filters out hosts based on its NUMA topology. If the instance has no topology defined, any host can be used. The filter tries to match the exact NUMA topology of the instance to those of the host (it does not attempt to pack the instance onto the host). The filter also looks at the standard over-subscription limits for each NUMA node, and provides limits to the compute host accordingly.

RetryFilter

Filters out hosts that have failed a scheduling attempt; valid if scheduler_max_attempts is greater than zero (defaults to "3").

SameHostFilter

Passes one or more specified hosts; specify hosts for the instance using the --hint same_host option for nova boot.

ServerGroupAffinityFilter

Only passes hosts for a specific server group:

  • Give the server group the affinity policy (nova server-group-create --policy affinity groupName).
  • Build the instance with that group (nova boot option --hint group=UUID)

ServerGroupAntiAffinityFilter

Only passes hosts in a server group that do not already host an instance:

  • Give the server group the anti-affinity policy (nova server-group-create --policy anti-affinity groupName).
  • Build the instance with that group (nova boot option --hint group=UUID).

SimpleCIDRAffinityFilter

Only passes hosts on the specified IP subnet range specified by the instance’s cidr and build_new_host_ip hints. Example:

--hint build_near_host_ip=192.0.2.0 --hint cidr=/24

4.5.2. Configuring scheduling weights

Hosts can be weighted for scheduling; the host with the largest weight (after filtering) is selected. All weighers are given a multiplier that is applied after normalising the node’s weight. A node’s weight is calculated as: 

w1_multiplier * norm(w1) + w2_multiplier * norm(w2) + ...

You can configure weight options in the Compute node’s configuration file.

Table 4.8. Configuration options for Scheduling service weights
Configuration optionDescription

filter_scheduler/weight_classes

Use this parameter to configure which of the following attributes to use for calculating the weight of each host:

  • nova.scheduler.weights.ram.RAMWeigher - Weighs the available RAM on the Compute node.
  • nova.scheduler.weights.cpu.CPUWeigher - Weighs the available CPUs on the Compute node.
  • nova.scheduler.weights.disk.DiskWeigher - Weighs the available disks on the Compute node.
  • nova.scheduler.weights.metrics.MetricsWeigher - Weighs the metrics of the Compute node.
  • nova.scheduler.weights.affinity.ServerGroupSoftAffinityWeigher - Weighs the proximity of the Compute node to other nodes in the given instance group.
  • nova.scheduler.weights.affinity.ServerGroupSoftAntiAffinityWeigher - Weighs the proximity of the Compute node to other nodes in the given instance group.
  • nova.scheduler.weights.compute.BuildFailureWeigher - Weighs Compute nodes by the number of recent failed boot attempts.
  • nova.scheduler.weights.io_ops.IoOpsWeigher - Weighs Compute nodes by their workload.
  • nova.scheduler.weights.pci.PCIWeigher - Weighs Compute nodes by their PCI availability.
  • nova.scheduler.weights.cross_cell.CrossCellWeigher - Weighs Compute nodes based on which cell they are in, giving preference to Compute nodes in the source cell when moving an instance.
  • nova.scheduler.weights.all_weighers - (Default) Uses all the above weighers.

Type: String

filter_scheduler/ram_weight_multiplier

Use this parameter to specify the multiplier to use to weigh hosts based on the available RAM.

Set to a positive value to prefer hosts with more available RAM, which spreads instances across many hosts.

Set to a negative value to prefer hosts with less available RAM, which fills up (stacks) hosts as much as possible before scheduling to a less-used host.

The absolute value, whether positive or negative, controls how strong the RAM weigher is relative to other weighers.

By default, the scheduler spreads instances across all hosts evenly (ram_weight_multiplier=1.0).

Type: Floating point

filter_scheduler/disk_weight_multiplier

Use this parameter to specify the multiplier to use to weigh hosts based on the available disk space.

Set to a positive value to prefer hosts with more available disk space, which spreads instances across many hosts.

Set to a negative value to prefer hosts with less available disk space, which fills up (stacks) hosts as much as possible before scheduling to a less-used host.

The absolute value, whether positive or negative, controls how strong the disk weigher is relative to other weighers.

By default, the scheduler spreads instances across all hosts evenly (disk_weight_multiplier=1.0).

Type: Floating point

filter_scheduler/cpu_weight_multiplier

Use this parameter to specify the multiplier to use to weigh hosts based on the available vCPUs.

Set to a positive value to prefer hosts with more available vCPUs, which spreads instances across many hosts.

Set to a negative value to prefer hosts with less available vCPUs, which fills up (stacks) hosts as much as possible before scheduling to a less-used host.

The absolute value, whether positive or negative, controls how strong the vCPU weigher is relative to other weighers.

By default, the scheduler spreads instances across all hosts evenly (cpu_weight_multiplier=1.0).

Type: Floating point

filter_scheduler/io_ops_weight_multiplier

Use this parameter to specify the multiplier to use to weigh hosts based on the host workload.

Set to a negative value to prefer hosts with lighter workloads, which distributes the workload across more hosts.

Set to a positive value to prefer hosts with heavier workloads, which schedules instances onto hosts that are already busy.

The absolute value, whether positive or negative, controls how strong the I/O operations weigher is relative to other weighers.

By default, the scheduler distributes the workload across more hosts (io_ops_weight_multiplier=-1.0).

Type: Floating point

filter_scheduler/build_failure_weight_multiplier

Use this parameter to specify the multiplier to use to weigh hosts based on recent build failures.

Set to a positive value to increase the significance of build failures recently reported by the host. Hosts with recent build failures are then less likely to be chosen.

Set to 0 to disable weighing compute hosts by the number of recent failures.

Default: 1000000.0

Type: Floating point

filter_scheduler/cross_cell_move_weight_multiplier

Use this parameter to specify the multiplier to use to weigh hosts during a cross-cell move. This option determines how much weight is placed on a host which is within the same source cell when moving an instance. By default, the scheduler prefers hosts within the same source cell when migrating an instance.

Set to a positive value to prefer hosts within the same cell the instance is currently running. Set to a negative value to prefer hosts located in a different cell from that where the instance is currently running.

Default: 1000000.0

Type: Floating point

filter_scheduler/pci_weight_multiplier

Use this parameter to specify the multiplier to use to weigh hosts based on the number of PCI devices on the host and the number of PCI devices requested by an instance. If an instance requests PCI devices, then the more PCI devices a Compute node has the higher the weight allocated to the Compute node.

For example, if there are three hosts available, one with a single PCI device, one with multiple PCI devices and one without any PCI devices, then the Compute scheduler prioritizes these hosts based on the demands of the instance. The first host should be preferred if the instance requests one PCI device, the second host if the instance requires multiple PCI devices and the third host if the instance does not request a PCI device.

Configure this option to prevent non-PCI instances from occupying resources on hosts with PCI devices.

Default: 1.0

Type: Positive floating point

filter_scheduler/host_subset_size

Use this parameter to specify the size of the subset of filtered hosts from which to select the host. Must be set to at least 1. A value of 1 selects the first host returned by the weighing functions. Any value less than 1 is ignored and 1 is used instead.

Set to a value greater than 1 to prevent multiple scheduler processes handling similar requests selecting the same host, creating a potential race condition. By selecting a host randomly from the N hosts that best fit the request, the chance of a conflict is reduced. However, the higher you set this value, the less optimal the chosen host may be for a given request.

Default: 1

Type: Integer

filter_scheduler/soft_affinity_weight_multiplier

Use this parameter to specify the multiplier to use to weigh hosts for group soft-affinity.

Default: 1.0

Type: Positive floating point

filter_scheduler/soft_anti_affinity_weight_multiplier

Use this parameter to specify the multiplier to use to weigh hosts for group soft-anti-affinity.

Default: 1.0

Type: Positive floating point

metrics/weight_multiplier

Use this parameter to specify the multiplier to use for weighting metrics. By default, weight_multiplier=1.0, which spreads instances across possible hosts.

Set to a number greater than 1.0 to increase the effect of the metric on the overall weight.

Set to a number between 0.0 and 1.0 to reduce the effect of the metric on the overall weight.

Set to 0.0 to ignore the metric value and return the value of the ‘weight_of_unavailable’ option.

Set to a negative number to prioritize the host with lower metrics, and stack instances in hosts.

Default: 1.0

Type: Floating point

metrics/weight_setting

Use this parameter to specify the metrics to use for weighting, and the ratio to use to calculate the weight of each metric. Valid metric names:

  • cpu.frequency - CPU frequency
  • cpu.user.time - CPU user mode time
  • cpu.kernel.time - CPU kernel time
  • cpu.idle.time - CPU idle time
  • cpu.iowait.time - CPU I/O wait time
  • cpu.user.percent - CPU user mode percentage
  • cpu.kernel.percent - CPU kernel percentage
  • cpu.idle.percent - CPU idle percentage
  • cpu.iowait.percent - CPU I/O wait percentage
  • cpu.percent - Generic CPU utilization

Example: weight_setting=cpu.user.time=1.0

Type: Comma-separated list of metric=ratio pairs.

metrics/required

Use this parameter to specify how to handle configured metrics/weight_setting metrics that are unavailable:

  • True - Metrics are required. If the metric is unavailable, an exception is raised. To avoid the exception, use the MetricsFilter filter in NovaSchedulerDefaultFilters.
  • False - The unavailable metric is treated as a negative factor in the weighing process. Set the returned value by using the weight_of_unavailable configuration option.

Type: Boolean

metrics/weight_of_unavailable

Use this parameter to specify the weight to use if any metrics/weight_setting metric is unavailable, and metrics/required=False.

Default: -10000.0

Type: Floating point

4.5.3. Reserving NUMA nodes with PCI devices

Compute uses the filter scheduler to prioritize hosts with PCI devices for instances requesting PCI. The hosts are weighted using the PCIWeigher option, based on the number of PCI devices available on the host and the number of PCI devices requested by an instance. If an instance requests PCI devices, then the hosts with more PCI devices are allocated a higher weight than the others. If an instance is not requesting PCI devices, then prioritization does not take place.

This feature is especially useful in the following cases:

  • As an operator, if you want to reserve nodes with PCI devices (typically expensive and with limited resources) for guest instances that request them.
  • As a user launching instances, you want to ensure that PCI devices are available when required.
Note

For this value to be considered, one of the following values must be added to the NovaSchedulerDefaultFilters parameter in your Compute environment file: PciPassthroughFilter or NUMATopologyFilter.

The pci_weight_multiplier configuration option must be a positive value.

4.6. Managing instance snapshots

You can use an instance snapshot to create a new image from an instance. This is very convenient for upgrading base images or for taking a published image and customizing it for local use.

The difference between an image that you upload directly to the Image service and an image that you create by snapshot is that an image created by snapshot has additional properties in the Image service database. These properties are in the image_properties table and include the following parameters:

Table 4.9. Snapshot options
NameValue

image_type

snapshot

instance_uuid

<uuid_of_instance_that_was_snapshotted>

base_image_ref

<uuid_of_original_image_of_instance_that_was_snapshotted>

image_location

snapshot

Use snapshots to create new instances based on that snapshot, and potentially restore an instance to that state. You can perform this action while the instance is running.

By default, a snapshot is accessible to the users and projects that were selected while launching an instance that the snapshot is based on.

4.6.1. Creating an instance snapshot

Note

If you intend to use an instance snapshot as a template to create new instances, you must ensure that the disk state is consistent. Before you create a snapshot, set the snapshot image metadata property os_require_quiesce=yes: 

$ openstack image set --property os_require_quiesce=yes <image_id>

For this to work, the guest must have the qemu-guest-agent package installed, and the image must be created with the metadata property parameter hw_qemu_guest_agent=yes set.: 

$ openstack image create \
--disk-format raw \
--container-format bare \
--file <file_name> \
--is-public True \
--property hw_qemu_guest_agent=yes \
--progress \
--name <name>

If you unconditionally enable the hw_qemu_guest_agent=yes parameter, then you are adding another device to the guest. This consumes a PCI slot, and limits the number of other devices you can allocate to the guest. It also causes Windows guests to display a warning message about an unknown hardware device.

For these reasons, setting the hw_qemu_guest_agent=yes parameter is optional, and you must use the parameter only for images that require the QEMU guest agent.

  1. In the dashboard, select Project > Compute > Instances.
  2. Select the instance from which you want to create a snapshot.
  3. In the Actions column, click Create Snapshot.

  4. In the Create Snapshot dialog, enter a name for the snapshot and click Create Snapshot.

    The Images category now shows the instance snapshot.

To launch an instance from a snapshot, select the snapshot and click Launch.

4.6.2. Managing a snapshot

  1. In the dashboard, select Project > Images.
  2. All snapshots you created, appear under the Project option.

  3. For every snapshot you create, you can perform the following functions, using the dropdown list:

    1. Use the Create Volume option to create a volume and entering the values for volume name, description, image source, volume type, size and availability zone. For more information, see Create a Volume in the Storage Guide.
    2. Use the Edit Image option to update the snapshot image by updating the values for name, description, Kernel ID, Ramdisk ID, Architecture, Format, Minimum Disk (GB), Minimum RAM (MB), public or private. For more information, see Section 1.2.3, “Updating an image”.
    3. Use the Delete Image option to delete the snapshot.

4.6.3. Rebuilding an instance to a state in a snapshot

In an event that you delete an instance on which a snapshot is based, the snapshot still stores the instance ID. You can check this information by using the nova image-list command and use the snapshot to restore the instance.

  1. In the dashboard, select Project > Compute > Images.
  2. Select the snapshot from which you want to restore the instance.
  3. In the Actions column, click Launch Instance.
  4. In the Launch Instance dialog, enter a name and the other details for the instance and click Launch.

For more information on launching an instance, see Section 4.1.2, “Launching an instance”.

4.6.4. Consistent snapshots

Previously, file systems had to be quiesced manually (fsfreeze) before taking a snapshot of active instances for consistent backups.

The Compute libvirt driver automatically requests the QEMU Guest Agent to freeze the file systems (and applications if fsfreeze-hook is installed) during an image snapshot. Support for quiescing file systems enables scheduled, automatic snapshots at the block device level.

This feature is valid only if the QEMU Guest Agent is installed (qemu-ga) and the image metadata enables the agent (hw_qemu_guest_agent=yes).

Note

Do not use snapshots as a substitute for system backups.

4.7. Using rescue mode for instances

Compute has a method to reboot a virtual machine in rescue mode. Rescue mode provides a mechanism for access when the virtual machine image renders the instance inaccessible. A rescue virtual machine allows a user to fix their virtual machine by accessing the instance with a new root password. This feature is useful if the file system of an instance is corrupted. By default, rescue mode starts an instance from the initial image attaching the current boot disk as a secondary one.

4.7.1. Preparing an image for a rescue mode instance

Due to the fact that both the boot disk and the disk for rescue mode have same UUID, sometimes the virtual machine can be booted from the boot disk instead of the disk for rescue mode.

To avoid this issue, you should create a new image as rescue image based on the procedure in Section 1.2.1, “Creating an image”:

Note

The rescue image is stored in glance and configured in the nova.conf as a default, or you can select when you do the rescue.

4.7.1.1. Rescuing an image that uses ext4 file system

When the base image uses ext4 file system, you can create a rescue image from it by using the following procedure:

  1. Change the UUID to a random value by using the tune2fs command: 

    # tune2fs -U random /dev/<device_node>

    Replace <device_node> with the root device node, for example, sda or vda.

  2. Verify the details of the file system, including the new UUID: 

    # tune2fs -l
  3. Update the /etc/fstab to use the new UUID. You might need to repeat this for any additional partitions that you have that are mounted in the fstab by UUID.
  4. Update the /boot/grub2/grub.conf file and update the UUID parameter with the new UUID of the root disk.
  5. Shut down and use this image as your rescue image. This causes the rescue image to have a new random UUID that does not conflict with the instance that you are rescuing.
Note

The XFS file system cannot change the UUID of the root device on the running virtual machine. Reboot the virtual machine until the virtual machine is launched from the disk for rescue mode.

4.7.2. Adding the rescue image to the OpenStack Image service

When you have completed modifying the UUID of your image, use the following commands to add the generated rescue image to the OpenStack Image service:

  1. Add the rescue image to the Image service: 

    # openstack image create --name <image_name> --disk-format qcow2 \
  --container-format bare --is-public True --file <image_path>

    Replace <image_name> with the name of the image and <image_path> with the location of the image.

  2. Use the image list command to obtain the <image_id> required to launch an instance in the rescue mode. 

    # openstack image list

You can also upload an image by using the OpenStack Dashboard, see Section 1.2.2, “Uploading an image”.

4.7.3. Launching an instance in rescue mode

  1. Because you need to rescue an instance with a specific image, rather than the default one, use the --image parameter: 

    # openstack server rescue --image <image> <instance>
    • Replace <image> with the name or ID of the image you want to use.
    • Replace <instance> with the name or ID of the instance that you want to rescue.
    Note

    For more information on rescuing an instance, see https://access.redhat.com/documentation/en-us/red_hat_openstack_platform/16.1/html/instances_and_images_guide/assembly-managing-an-instance_instances#rescuing-an-instance_instances

    By default, the instance has 60 seconds to shut down. You can override the timeout value on a per image basis by using the image metadata setting os_shutdown_timeout to specify the time that different types of operating systems require to shut down cleanly.

  2. Reboot the virtual machine.
  3. Confirm the status of the virtual machine is RESCUE on the controller node by using nova list command or by using dashboard.
  4. Log in to the new virtual machine dashboard by using the password for rescue mode.

You can now make the necessary changes to your instance to fix any issues.

4.7.4. Unrescuing an instance

You can unrescue the fixed instance to restart it from the boot disk.

  1. Execute the following commands on the controller node. 

    # nova unrescue <virtual_machine_id>

    Here <virtual_machine_id> is ID of a virtual machine that you want to unrescue.

The status of your instance returns to ACTIVE after the unrescue operation has completed successfully. :leveloffset: +3

4.8. Creating a customized instance

Cloud users can specify additional data to use when they launch an instance, such as a shell script that the instance runs on boot. The cloud user can use the following methods to pass data to instances:

User data
Use to include instructions in the instance launch command for cloud-init to execute.
Instance metadata
A list of key-value pairs that you can specify when you create or update an instance.

You can access the additional data passed to the instance by using a config drive or the metadata service.

Config drive
You can attach a config drive to an instance when it boots. The config drive is presented to the instance as a read-only drive. The instance can mount this drive and read files from it. You can use the config drive as a source for cloud-init information. Config drives are useful when combined with cloud-init for server bootstrapping, and when you want to pass large files to your instances. For example, you can configure cloud-init to automatically mount the config drive and run the setup scripts during the initial instance boot. Config drives are created with the volume label of config-2, and attached to the instance when it boots. The contents of any additional files passed to the config drive are added to the user_data file in the openstack/{version}/ directory of the config drive. cloud-init retrieves the user data from this file.
Metadata service
Uses a REST API to retrieve data specific to an instance. Instances access this service at 169.254.169.254 or at fe80::a9fe:a9fe.

cloud-init can use both a config drive and the metadata service to consume the additional data for customizing an instance. The cloud-init package supports several data input formats. Shell scripts and the cloud-config format are the most common input formats:

  • Shell scripts: The data declaration begins with #! or Content-Type: text/x-shellscript. Shell scripts are invoked last in the boot process.
  • cloud-config format: The data declaration begins with #cloud-config or Content-Type: text/cloud-config. cloud-config files must be valid YAML to be parsed and executed by cloud-init.
Note

cloud-init has a maximum user data size of 16384 bytes for data passed to an instance. You cannot change the size limit, therefore use gzip compression when you need to exceed the size limit.

4.8.1. Customizing an instance by using user data

You can use user data to include instructions in the instance launch command. cloud-init executes these commands to customize the instance as the last step in the boot process.

Procedure

  1. Create a file with instructions for cloud-init. For example, create a bash script that installs and enables a web server on the instance: 

    $ vim /home/scripts/install_httpd
#!/bin/bash

yum -y install httpd python-psycopg2
systemctl enable httpd --now

  2. Launch an instance with the --user-data option to pass the bash script: 

    $ openstack server create \
--image rhel8 \
--flavor default \
--nic net-id=web-server-network \
--security-group default \
--key-name web-server-keypair \
--user-data /home/scripts/install_httpd \
--wait web-server-instance

  3. When the instance state is active, attach a floating IP address: 

    $ openstack floating ip create web-server-network
$ openstack server add floating ip web-server-instance 172.25.250.123

  4. Log in to the instance with SSH: 

    $ ssh -i ~/.ssh/web-server-keypair cloud-user@172.25.250.123

  5. Check that the customization was successfully performed. For example, to check that the web server has been installed and enabled, enter the following command: 

    $ curl http://localhost | grep Test
<title>Test Page for the Apache HTTP Server on Red Hat Enterprise Linux</title>
<h1>Red Hat Enterprise Linux <strong>Test Page</strong></h1>

  6. Review the /var/log/cloud-init.log file for relevant messages, such as whether or not the cloud-init executed: 

    $ sudo less /var/log/cloud-init.log
...output omitted...
...util.py[DEBUG]: Cloud-init v. 0.7.9 finished at Sat, 23 Jun 2018 02:26:02 +0000. Datasource DataSourceOpenStack [net,ver=2].  Up 21.25 seconds

4.8.2. Customizing an instance by using metadata

You can use instance metadata to specify the properties of an instance in the instance launch command.

Procedure

  1. Launch an instance with the --property <key=value> option. For example, to mark the instance as a webserver, set the following property: 

    $ openstack server create \
--image rhel8 \
--flavor default \
--property role=webservers \
--wait web-server-instance

  2. Optional: Add an additional property to the instance after it is created, for example: 

    $ openstack server set \
--property region=emea \
--wait web-server-instance

4.8.3. Customizing an instance by using a config drive

You can create a config drive for an instance that is attached during the instance boot process. You can pass content to the config drive that the config drive makes available to the instance.

Procedure

  1. Enable the config drive, and specify a file that contains content that you want to make available in the config drive. For example, the following command creates a new instance named config-drive-instance and attaches a config drive that contains the contents of the file my-user-data.txt: 

    (overcloud)$ openstack server create --flavor m1.tiny \
  --config-drive true \
  --user-data ./my-user-data.txt \
  --image cirros config-drive-instance

    This command creates the config drive with the volume label of config-2, which is attached to the instance when it boots, and adds the contents of my-user-data.txt to the user_data file in the openstack/{version}/ directory of the config drive.

  2. Log in to the instance.

  3. Mount the config drive:

    • If the instance OS uses udev: 

      # mkdir -p /mnt/config
# mount /dev/disk/by-label/config-2 /mnt/config

    • If the instance OS does not use udev, you need to first identify the block device that corresponds to the config drive: 

      # blkid -t LABEL="config-2" -odevice
/dev/vdb
# mkdir -p /mnt/config
# mount /dev/vdb /mnt/config
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