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DM Multipath

Red Hat Enterprise Linux 4

DM Multipath Configuration and Administration

Edition 1.0

Abstract

This book provides information on using the Device-Mapper Multipath feature of Red Hat Enterprise Linux 4.

Preface

This book describes the Device Mapper Multipath (DM-Multipath) feature of Red Hat Enterprise Linux.

1. Audience

This book is intended to be used by system administrators managing systems running the Linux operating system. It requires familiarity with Red Hat Enterprise Linux.

3. Feedback

If you spot a typo, or if you have thought of a way to make this manual better, we would love to hear from you. Please submit a report in Bugzilla (http://bugzilla.redhat.com/bugzilla/) against the component rh-cs.
Be sure to mention the manual's identifier: 
rh-MPIO(EN)-4.9 (2011-02-16T16:48)
By mentioning this manual's identifier, we know exactly which version of the guide you have.
If you have a suggestion for improving the documentation, try to be as specific as possible. If you have found an error, please include the section number and some of the surrounding text so we can find it easily.

Chapter 1. Device Mapper Multipathing

Device Mapper Multipathing (DM-Multipath) allows you to configure multiple I/O paths between server nodes and storage arrays into a single device. These I/O paths are physical SAN connections that can include separate cables, switches, and controllers. Multipathing aggregates the I/O paths, creating a new device that consists of the aggregated paths.

1.1. Overview of DM-Multipath

DM-Multipath can be used to provide:
  • Redundancy
    DM-Multipath can provide failover in an active/passive configuration. In an active/passive configuration, only half the paths are used at any time for I/O. If any element of an I/O path (the cable, switch, or controller) fails, DM-Multipath switches to an alternate path.
  • Improved Performance
    DM-Multipath can be configured in active/active mode, where I/O is spread over the paths in a round-robin fashion. In some configurations, DM-Multipath can detect loading on the I/O paths and dynamically re-balance the load.
Figure 1.1, “Active/Passive Multipath Configuration with One RAID Device” shows an active/passive configuration with two I/O paths from the server to a RAID device. There are 2 HBAs on the server, 2 SAN switches, and 2 RAID controllers.
Active/Passive Multipath Configuration with One RAID Device

Figure 1.1. Active/Passive Multipath Configuration with One RAID Device

In this configuration, there is one I/O path that goes through hba1, SAN1, and controller 1 and a second I/O path that goes through hba2, SAN2, and controller2. There are many points of possible failure in this configuration:
  • HBA failure
  • FC cable failure
  • SAN switch failure
  • Array controller port failure
With DM-Multipath configured, a failure at any of these points will cause DM-Multipath to switch to the alternate I/O path.
Figure 1.2, “Active/Passive Multipath Configuration with Two RAID Devices” shows a more complex active/passive configuration with 2 HBAs on the server, 2 SAN switches, and 2 RAID devices with 2 RAID controllers each.
Active/Passive Multipath Configuration with Two RAID Devices

Figure 1.2. Active/Passive Multipath Configuration with Two RAID Devices

As in the example shown in Figure 1.1, “Active/Passive Multipath Configuration with One RAID Device”, there are two I/O paths to each RAID device. With DM-Multipath configured, a failure at any of the points of the I/O path to either of the RAID devices will cause DM-Multipath to switch to the alternate I/O path for that device.
Figure 1.3, “Active/Active Multipath Configuration with One RAID Device” shows an active/active configuration with 2 HBAs on the server, 1 SAN switch, and 2 RAID controllers. There are four I/O paths from the server to a storage device:
  • hba1 to controller1
  • hba1 to controller2
  • hba2 to controller1
  • hba2 to controller2
In this configuration, I/O can be spread among those four paths.
Active/Active Multipath Configuration with One RAID Device

Figure 1.3. Active/Active Multipath Configuration with One RAID Device

1.2. Storage Array Support

By default, DM-Multipath includes support for the most common storage arrays that support DM-Multipath. The supported devices can be found in the multipath.conf.defaults file. If your storage array supports DM-Multipath and is not configured by default in this file, you may need to add them to the DM-Multipath configuration file, multipath.conf. For information on the DM-Multipath configuration file, see Chapter 4, The DM-Multipath Configuration File.
Some storage arrays require special handling of I/O errors and path switching. These require separate hardware handler kernel modules.

1.3. DM-Multipath Components

Table 1.1, “DM-Multipath Components”. describes the components of DM-Multipath.
Table 1.1. DM-Multipath Components
Component Description
dm-multipath kernel module Reroutes I/O and supports failover for paths and path groups.
multipath command Lists and configures multipath devices. Normally started up with /etc/rc.sysinit, it can also be started up by a udev program whenever a block device is added or it can be run by the initramfs file system.
multipathd daemon Monitors paths; as paths fail and come back, it may initiate path group switches. Provides for interactive changes to multipath devices. This must be restarted for any changes to the /etc/multipath.conf file.
kpartx command Creates device mapper devices for the partitions on a device It is necessary to use this command for DOS-based partitions with DM-MP. The kpartx is provided in its own package, but the device-mapper-multipath package depends on it.

1.4. DM-Multipath Setup Overview

DM-Multipath includes compiled-in default settings that are suitable for common multipath configurations. Setting up DM-multipath is often a simple procedure.
The basic procedure for configuring your system with DM-Multipath is as follows:
  1. Install device-mapper-multipath rpm.
  2. Edit the multipath.conf configuration file:
    • comment out the default blacklist
    • change any of the existing defaults as needed
    • save the configuration file
  3. Start the multipath daemons.
  4. Create the multipath device with the multipath command.
Detailed setup instructions for several example multipath configurations are provided in see Chapter 3, Setting Up DM-Multipath.

Chapter 2. Multipath Devices

Without DM-Multipath, each path from a server node to a storage controller is treated by the system as a separate device, even when the I/O path connects the same server node to the same storage controller. DM-Multipath provides a way of organizing the I/O paths logically, by creating a single multipath device on top of the underlying devices.

2.1. Multipath Device Identifiers

Each multipath device has a World Wide Identifier (WWID), which is guaranteed to be globally unique and unchanging. By default, the name of a multipath device is set to its WWID. Alternately, you can set the user_friendly_names option in the multipath configuration file, which sets the alias to a node-unique name of the form mpathn.
For example, a node with two HBAs attached to a storage controller with two ports via a single unzoned FC switch sees four devices: /dev/sda, /dev/sdb, dev/sdc, and /dev/sdd. DM-Multipath creates a single device with a unique WWID that reroutes I/O to those four underlying devices according to the multipath configuration. When the user_friendly_names configuration option is set to yes, the name of the multipath device is set to mpathn.
When new devices are brought under the control of DM-Multipath, the new devices may be seen in three different places under the /dev directory: /dev/mapper/mpathn, /dev/mpath/mpathn, and /dev/dm-n.
  • The devices in /dev/mapper are created early in the boot process. Use these devices to access the multipathed devices, for example when creating logical volumes.
  • The devices in /dev/mpath are provided as a convenience so that all multipathed devices can be seen in one directory. These devices are created by the udev device manager and may not be available on startup when the system needs to access them. Do not use these devices for creating logical volumes or filesystems.
  • Any devices of the form /dev/dm-n are for internal use only and should never be used.
For information on the multipath configuration defaults, including the user_friendly_names configuration option, see see Section 4.3, “Configuration File Defaults”.
You can also set the name of a multipath device to a name of your choosing by using the alias option in the multipaths section of the multipath configuration file. For information on the multipaths section of the multipath configuration file, see see Section 4.4, “Multipaths Device Configuration Attributes”.

2.2. Consistent Multipath Device Names in a Cluster

When the user_friendly_names configuration option is set to yes, the name of the multipath device is unique to a node, but it is not guaranteed to be the same on all nodes using the multipath device. This should not cause any difficulties if you use LVM to create logical devices from the multipath device, but if you require that your multipath device names be consistent in every node in the cluster you perform one of the following procedures:
  • Use the alias option in the multipaths section of the multipath configuration file to set the name of the multipath device. The alias for the multipath device is consistent across all the nodes in a cluster. For information on the multipaths section of the multipath configuration file, see see Section 4.4, “Multipaths Device Configuration Attributes”.
  • If you want the system-defined user-friendly names to be consistent across all nodes in the cluster, set up all of the multipath devices on one machine. Then copy the bindings file from that machine to all the other machines in the cluster. The bindings file is located at /var/lib/multipath/bindings by default, but as of RHEL 4.6 and later you can set this value to a different location with the bindings_file parameter of the defaults section of the configuration file.

2.3. Multipath Device Attributes

In addition to the user_friendly_names and alias options, a multipath device has numerous attributes. You can modify these attributes for a specific multipath device by creating an entry for that device in the multipaths section of the multipath configuration file. For information on the multipaths section of the multipath configuration file, see see Section 4.4, “Multipaths Device Configuration Attributes”.

2.4. Multipath Devices in Logical Volumes

After creating multipath devices, you can use the multipath device names just as you would use a physical device name when creating an LVM physical volume. For example, if /dev/mapper/mpath0 is the name of a multipath device, the following command will mark /dev/mapper/mpath0 as a physical volume. 
pvcreate /dev/mapper/mpath0
You can use the resulting LVM physical device when you create an LVM volume group just as you would use any other LVM physical device.
When you create an LVM logical volume that uses active/passive multipath arrays as the underlying physical devices, you should include filters in the lvm.conf file to exclude the disks that underlie the multipath devices. This is because if the array automatically changes the active path to the passive path when it receives I/O, multipath will failover and failback whenever LVM scans the passive path if these devices are not filtered. For active/passive arrays that require a command to make the passive path active, LVM prints a warning message when this occurs.
To filter all SCSI devices in the multipath configuration file (lvm.conf), include the following filter in the devices section of the file. 
filter = [ "r/disk/", "r/sd.*/", "a/.*/" ]

Chapter 3. Setting Up DM-Multipath

This chapter provides step-by-step example procedures for configuring DM-Multipath. It includes the following procedures:
  • Basic DM-Multipath setup
  • Ignoring local disks
  • Adding more devices to the configuration file

3.1. Setting Up DM-Multipath

Before setting up DM-Multipath on your system, ensure that your system has been updated and includes the device-mapper-multipath package.
Use the following procedure to set up DM-Multipath for a basic failover configuration.
  1. Edit the /etc/multipath.conf file by commenting out the following lines at the top of the file. This section of the configuration file, in its initial state, blacklists all devices. You must comment it out to enable multipathing. 
    devnode_blacklist {
        devnode "*"
}
    After commenting out those lines, this section appears as follows. 
    # devnode_blacklist {
#        devnode "*"
# }
  2. The default settings for DM-Multipath are compiled in to the system and do not need to be explicitly set in the /etc/multipath.conf file.
    The default value of path_grouping_policy is set to failover, so in this example you do not need to change the default value. For information on changing the values in the configuration file to something other than the defaults, see Chapter 4, The DM-Multipath Configuration File.
    The initial defaults section of the configuration file configures your system that the names of the multipath devices are are of the form mpathn; without this setting, the names of the multipath devices would be aliased to the WWID of the device.
  3. Save the configuration file and exit the editor.
  4. Execute the following commands: 
    modprobe dm-multipath
service multipathd start
multipath -v2
    The multipath -v2 command prints out multipathed paths that show which devices are multipathed, but only for the devices created by this command. If the command does yield any output, you can check your multipath devices as follows:
    • Run the multipath -ll command. This lists all the multipath devices.
    • If running the multipath -ll command does not show the device, verify that multipath is configured properly by checking the /etc/multipath file and making sure that the SCSI devices you want to be multipathed exist on the system.
    • If the SCSI devices do not appear, ensure that all SAN connections are set up properly.
    For further information on the multipath command and its output, see Section 5.1, “Multipath Command Output”, see Section 5.2, “Multipath Queries with multipath Command”, and see Section 5.3, “Multipath Command Options”.
  5. Execute the following command to ensure sure that the multipath daemon starts on bootup: 
    chkconfig multipathd on
Since the value of user_friendly_name is set to yes in the configuration filea the multipath devices will be created as /dev/mapper/mpathn. For information on setting the name of the device to an alias of your choosing, see Chapter 4, The DM-Multipath Configuration File.

3.2. Ignoring Local Disks when Generating Multipath Devices

Some machines have local SCSI cards for their internal disks. DM-Multipath is not recommended for these devices. The following procedure shows how to modify the multipath configuration file to ignore the local disks when configuring multipath.
  1. Determine which disks are the internal disks and mark them as the ones to blacklist.
    In this example, /dev/sda is the internal disk. Note that as originally configured in the default multipath configuration file, executing the multipath -v2 shows the local disk, /dev/sda, in the multipath map.
    For further information on the multipath command output, see Section 5.1, “Multipath Command Output”. 
    [root@rh4cluster1 ~]# multipath -v2
create: SIBM-ESXSST336732LC____F3ET0EP0Q000072428BX1
[size=33 GB][features="0"][hwhandler="0"]
\_ round-robin 0 
  \_ 0:0:0:0 sda  8:0    [--------- 

device-mapper ioctl cmd 9 failed: Invalid argument
device-mapper ioctl cmd 14 failed: No such device or address
create: 3600a0b80001327d80000006d43621677
[size=12 GB][features="0"][hwhandler="0"]
\_ round-robin 0 
  \_ 2:0:0:0 sdb  8:16    
  \_ 3:0:0:0 sdf  8:80    

create: 3600a0b80001327510000009a436215ec
[size=12 GB][features="0"][hwhandler="0"]
\_ round-robin 0 
  \_ 2:0:0:1 sdc  8:32    
  \_ 3:0:0:1 sdg  8:96    

create: 3600a0b80001327d800000070436216b3
[size=12 GB][features="0"][hwhandler="0"]
\_ round-robin 0 
  \_ 2:0:0:2 sdd  8:48    
  \_ 3:0:0:2 sdh  8:112   

create: 3600a0b80001327510000009b4362163e
[size=12 GB][features="0"][hwhandler="0"]
\_ round-robin 0 
  \_ 2:0:0:3 sde  8:64    
  \_ 3:0:0:3 sdi  8:128
  2. In order to prevent the device mapper from mapping /dev/sda in its multipath maps, edit the devnode_blacklist section of the /etc/multipath.conf file to include this device. Although you could blacklist the sda device using a devnode type, that would not be safe procedure since /dev/sda is not guaranteed to be the same on reboot. To blacklist individual devices, you can blacklist using the WWID of that device.
    Note that in the output to the multipath -vs command, the WWID of the /dev/sda device is SIBM-ESXSST336732LC____F3ET0EP0Q000072428BX1. To blacklist this device, include the following in the /etc/multipath.conf file. 
    devnode_blacklist {
      wwid SIBM-ESXSST336732LC____F3ET0EP0Q000072428BX1
}
  3. Run the following commands: 
    multipath -F
multipath -v2
    The local disk or disks should no longer be listed in the new multipath maps, as shown in the following example. 
    [root@rh4cluster1 ~]# multipath -F
[root@rh4cluster1 ~]# multipath -v2
create: 3600a0b80001327d80000006d43621677
[size=12 GB][features="0"][hwhandler="0"]
\_ round-robin 0 
  \_ 2:0:0:0 sdb  8:16    
  \_ 3:0:0:0 sdf  8:80    

create: 3600a0b80001327510000009a436215ec
[size=12 GB][features="0"][hwhandler="0"]
\_ round-robin 0 
  \_ 2:0:0:1 sdc  8:32    
  \_ 3:0:0:1 sdg  8:96    

create: 3600a0b80001327d800000070436216b3
[size=12 GB][features="0"][hwhandler="0"]
\_ round-robin 0 
  \_ 2:0:0:2 sdd  8:48    
  \_ 3:0:0:2 sdh  8:112   

create: 3600a0b80001327510000009b4362163e
[size=12 GB][features="0"][hwhandler="0"]
\_ round-robin 0 
  \_ 2:0:0:3 sde  8:64    
  \_ 3:0:0:3 sdi  8:128

3.3. Adding Devices to the Multipathing Database

By default, DM-Multipath includes support for the most common storage arrays that support DM-Multipath. The default configuration values, including supported devices, can be found in the multipath.conf.defaults file.
If you need to add a storage device that is not supported by default as a known multipath device, edit the /etc/multipath.conf file and insert the appropriate device information.
For example, to add information about the HP Open-V series the entry looks like this: 
devices {
       device {
               vendor "HP"
               product "OPEN-V."
               getuid_callout "/sbin/scsi_id -g -u -p0x80 -s /block/%n"
       }
}
For more information on the devices section of the configuration file, see Section 4.5, “Configuration File Devices”.

Chapter 4. The DM-Multipath Configuration File

By default, DM-Multipath provides configuration values for the most common uses of multipathing. In addition, DM-Multipath includes support for the most common storage arrays that support DM-Multipath. The default configuration values and the supported devices can be found in the /usr/share/doc/device-mapper-multipath-0.4.5/multipath.conf.defaults file.

Note

The location and contents of the multipath.conf.defaults file may differ slightly from the example shown in this chapter depending on the release of RHEL 4 that you are running.
You can override the default configuration values for DM-Multipath by editing the /etc/multipath.conf configuration file. If necessary, you can also add a storage array that is not supported by default to the configuration file. This chapter provides information on parsing and modifying the multipath.conf file. It contains sections on the following topics:
  • Configuration file overview
  • Configuration file blacklist
  • Configuration file defaults
  • Configuration file multipaths
  • Configuration file devices
In the multipath configuration file, you need to specify only the sections that you need for your configuration, or that you wish to change from the default values specified in the multipath.conf.defaults file. If there are sections of the file that are not relevant to your environment or for which you do not need to override the default values, you can leave them commented out, as they are in the initial file.
The configuration file allows regular expression description syntax.
An annotated version of the configuration file can be found in /usr/share/doc/device-mapper-multipathd-0.4.5/multipath.conf.annotated.

4.1. Configuration File Overview

The multipath configuration file is divided into the following sections:
devnode_blacklist
Listing of specific devices that will not be considered for multipath. By default all devices are blacklisted. Usually the default devnode_blacklist section is commented out.
defaults
General default settings for DM-Multipath.
multipaths
Settings for the characteristics of individual multipath devices. These values overwrite what is specified in the defaults and devices sections of the configuration file.
devices
Settings for the individual storage controllers. These values overwrite what is specified in the defaults section of the configuration file. If you are using a storage array that is not supported by default, you may need to create a devices subsection for your array.
When the system determines the attributes of a multipath device, first it checks the multipath settings, then the per devices settings, then the multipath system defaults.

4.2. Configuration File Blacklist

The devnode_blacklist section of the multipath configuration file specifies the devices that will not be used when the system configures multipath devices. Devices that are blacklisted will not be grouped into a multipath device.
By default, all devices are blacklisted, since the following lines appear in the initial configuration file. 
devnode_blacklist {
        devnode "*"
}
To enable multipathing on all of the devices that are supported by default, comment out those lines, as described in Section 3.1, “Setting Up DM-Multipath”.
After commenting out the universal blacklist, you can specify general device types and individual devices to blacklist. You can blacklist devices according to the following criteria:
By default, a variety of device types are blacklisted, even after you comment out the initial devnode_blacklist section of the configuration file. For information, see Section 4.2.2, “Blacklisting By Device Name”.

4.2.1. Blacklisting By WWID

You can specify individual devices to blacklist by their World-Wide IDentification with a wwid entry in the blacklist section of the configuration file.
The following example shows the lines in the configuration file that would blacklist a device with a WWID of 26353900f02796769. 
blacklist {
       wwid 26353900f02796769
}

4.2.2. Blacklisting By Device Name

You can blacklist device types by device name so that they will not be grouped into a multipath device by specifying a devnode entry in the devnode_blacklist section of the configuration file.
The following example shows the lines in the configuration file that would blacklist all SCSI devices, since it blacklists all sd* devices. 
devnode_blacklist {
       devnode "^sd[a-z]"
}
You can use a devnode entry in the blacklist section of the configuration file to specify individual devices to blacklist rather than all devices of specific type; this is not recommended, however. Unless it is statically mapped by udev rules, there is no guarantee that a specific device will have the same name on reboot. For example, a device name could change from /dev/sda to /dev/sdb on reboot.
By default, the following devnode entries are compiled in the default blacklist; the devices that these entries blacklist do not generally support DM-Multipath. 
blacklist {
       devnode "^(ram|raw|loop|fd|md|dm-|sr|scd|st)[0-9]*"
       devnode "^hd[a-z]"
       devnode "^cciss!c[0-9]d[0-9]*"
}

4.3. Configuration File Defaults

The /etc/multipath.conf configuration file includes a defaults section that sets the user_friendly_names parameter to yes, as follows. 
defaults {
        user_friendly_names yes
}
This overwrites the default value of the user_friendly_names parameter.
The configuration file includes a template of configuration defaults. This section is commented out, as follows. 
#defaults {
#       udev_dir                /dev
#       polling_interval        10
#       selector                "round-robin 0"
#       path_grouping_policy    multibus
#       getuid_callout          "/sbin/scsi_id -g -u -s /block/%n"
#       prio_callout            /bin/true
#       path_checker            readsector0
#       rr_min_io               100
#       rr_weight               priorities
#       failback                immediate
#       no_path_retry           fail
#       user_friendly_name      yes
#}
To overwrite the default value for any of the configuration parameters, you can copy the relevant line from this template into the defaults section and uncomment it. For example, to overwrite the path_grouping_policy parameter so that it is multibus rather than the default value of failover, copy the appropriate line from the template to the initial defaults section of the configuration file, and uncomment it, as follows. 
defaults {
        user_friendly_names     yes
        path_grouping_policy    multibus
}
Table 4.1, “Multipath Configuration Defaults” describes the attributes that are set in the defaults section of the multipath.conf configuration file. These values are used by DM-Multipath unless they are overwritten by the attributes specified in the devices and multipaths sections of the multipath.conf file.
Table 4.1. Multipath Configuration Defaults
Attribute Description
udev_dir Specifies the directory where udev device nodes are created. The default value is /udev.
polling_interval Specifies the interval between two path checks in seconds. The default value is 5.
selector Specifies the default algorithm to use in determining what path to use for the next I/O operation. The default value is round-robin 0.
path_grouping_policy Specifies the default path grouping policy to apply to unspecified multipaths. Possible values include:
failover = 1 path per priority group
multibus = all valid paths in 1 priority group
group_by_serial = 1 priority group per detected serial number
group_by_prio = 1 priority group per path priority value
group_by_node_name = 1 priority group per target node name
The default value is failover.
getuid_callout Specifies the default program and arguments to call out to obtain a unique path identifier. An absolute path is required.
The default value is /sbin/scsi_id -g -u -s.
prio_callout Specifies the default program and arguments to call out to obtain a path priority value. For example, the ALUA bits in SPC-3 provide an exploitable prio value for example. "none" is a valid value. The default value is no callout, indicating all paths are equal
features Specifies the default extra features of multipath devices. The only existing feature is queue_if_no_path. The default value is (null).
path_checker Specifies the default method used to determine the state of the paths. Possible values include: readsector0, tur, emc_clariion, hp_sw, and directio. The default value is readsector0.
failback Specifies path group failback.
A value of 0 or immediate specifies that as soon as there is a path group with a higher priority than the current path group the system switches to that path group.
A numeric value greater than zero specifies deferred failback, expressed in seconds.
A value of manual specifies that failback can happen only with operator intervention.
The default value is manual.
rr_min_io Specifies the number of I/O requests to route to a path before switching to the next path in the current path group. The default value is 1000.
max_fds (RHEL 4.7 and later) Sets the maximum number of open file descriptors for the multipathd process. A value of max sets the number of open file descriptors to the system maximum.
rr_weight If set to priorities, then instead of sending rr_min_io requests to a path before calling selector to choose the next path, the number of requests to send is determined by rr_min_io times the path's priority, as determined by the prio_callout program. Currently, there are priority callouts only for devices that use the group_by_prio path grouping policy, which means that all the paths in a path group will always have the same priority.
If set to uniform, all path weights are equal. The default value is uniform.
no_path_retry A numeric value for this attribute specifies the number of times the system should attempt to use a failed path before disabling queueing.
A value of fail indicates immediate failure, without queuing.
A value of queue indicates that queuing should not stop until the path is fixed.
The default value is (null).
flush_on_last_del (RHEL 4.7 and later) If set to yes, the multipathd daemon will disable queueing when the last path to a device has been deleted. The default value is no.
user_friendly_names If set to yes, specifies that the system should using the bindings file /var/lib/multipath/bindings to assign a persistent and unique alias to the multipath, in the form of mpathn. If set to no, specifies that the system should use use the WWID as the alias for the multipath. In either case, what is specified here will be overridden by any device-specific aliases you specify in the multipaths section of the configuration file. The default value is no.
bindings_file (RHEL 4.6 and later) The location of the bindings file that is used with the user_friendly_names option. The default value is /var/lib/multipath/bindings.
mode (RHEL 4.7 and later) The mode to use for the multipath device nodes, in octal. The default value is determined by the process.
uid (RHEL 4.7 and later) The user ID to use for the multipath device nodes. You must use the numeric user ID. The default value is determined by the process.
gid (RHEL 4.7 and later) The group ID to use for the multipath device nodes. You must use the numeric group ID. The default value is determined by the process.

4.4. Multipaths Device Configuration Attributes

Table 4.2, “Multipath Attributes” shows the attributes that you can set in the multipaths section of the multipath.conf configuration file for each specific multipath device. These attributes apply only to the one specified multipath. These defaults are used by DM-Multipath and override attributes set in the defaults and devices sections of the multipath.conf file.
Table 4.2. Multipath Attributes
Attribute Description
wwid Specifies the WWID of the multipath device to which the multipath attributes apply.
alias Specifies the symbolic name for the multipath device to which the multipath attributes apply.
path_grouping_policy Specifies the default path grouping policy to apply to unspecified multipaths. Possible values include:
failover = 1 path per priority group
multibus = all valid paths in 1 priority group
group_by_serial = 1 priority group per detected serial number
group_by_prio = 1 priority group per path priority value
group_by_node_name = 1 priority group per target node name
path_selector Specifies the default algorithm to use in determining what path to use for the next I/O operation.
rr_min_io (RHEL 4.8 and later) Specifies the number of I/O requests to route to a path before switching to the next path in the current path group.
failback Specifies path group failback.
A value of 0 or immediate specifies that as soon as there is a path group with a higher priority than the current path group the system switches to that path group.
A numeric value greater than zero specifies deferred failback, expressed in seconds.
A value of manual specifies that failback can happen only with operator intervention.
rr_weight If set to priorities, then instead of sending rr_min_io requests to a path before calling selector to choose the next path, the number of requests to send is determined by rr_min_io times the path's priority, as determined by the prio_callout program. Currently, there are priority callouts only for devices that use the group_by_prio path grouping policy, which means that all the paths in a path group will always have the same priority.
If set to uniform, all path weights are equal.
no_path_retry A numeric value for this attribute specifies the number of times the system should attempt to use a failed path before disabling queueing.
A value of fail indicates immediate failure, without queueing.
A value of queue indicates that queuing should not stop until the path is fixed.
flush_on_last_del (RHEL 4.7 and later) If set to yes, the multipathd daemon will disable queueing when the last path to a device has been deleted. The default value is no.
mode (RHEL 4.7 and later) The mode to use for the multipath device nodes, in octal. The default value is determined by the process.
uid (RHEL 4.7 and later) The user ID to use for the multipath device nodes. You must use the numeric user ID. The default value is determined by the process.
gid (RHEL 4.7 and later) The group ID to use for the multipath device nodes. You must use the numeric group ID. The default value is determined by the process.
The following example shows multipath attributes specified in the configuration file for two specific multipath devices. The first device has a WWID of 3600508b4000156d70001200000b0000 and a symbolic name of yellow.
The second multipath device in the example has a WWID of 1DEC_____321816758474 and a symbolic name of red. In this example, the rr_weight attributes is set to priorities. 
multipaths {
       multipath {
              wwid                  3600508b4000156d70001200000b0000
              alias                 yellow
              path_grouping_policy  multibus
              path_checker          readsector0
              path_selector         "round-robin 0"
              failback              manual
              rr_weight             priorities
              no_path_retry         5
       }
       multipath {
              wwid                  1DEC_____321816758474
              alias                 red
              rr_weight             priorities
        }
}

4.5. Configuration File Devices

Table 4.3, “Device Attributes” shows the attributes that you can set for each individual storage device in the devices section of the multipath.conf configuration file. These attributes are used by DM-Multipath unless they are overwritten by the attributes specified in the multipaths section of the multipath.conf file for paths that contain the device. These attributes override the attributes set in the defaults section of the multipath.conf file.
Many devices that support multipathing are included by default in a multipath configuration. The values for the devices that are supported by default are listed in the multipath.conf.defaults file. You probably will not need to modify the values for these devices, but if you do you can overwrite the default values by including an entry in the the configuration file for the device that overwrites those values. You can copy the device configuration defaults from the multipath.conf.defaults file for the device and override the values that you want to change.
To add a device to this section of the configuration file that is not configured automatically by default, you need to set the vendor and product parameters. You can find these values by looking at /sys/block/device_name/device/vendor and /sys/block/device_name/device/model where device_name is the device to be multipathed, as in the following example: 
[root@cypher-06 ~]# cat /sys/block/sda/device/vendor
WINSYS  
[root@cypher-06 ~]# cat /sys/block/sda/device/model
SF2372
The additional parameters to specify depend on your specific device. If the device is active/active, you will usually not need to set additional parameters. You may want to set path_grouping_policy to multibus. Other parameters you may need to set are no_path_retry and rr_min_io, as described in Table 4.3, “Device Attributes”.
If the device is active/passive, but it automatically switches paths with I/O to the passive path, you need to change the checker function to one that does not send I/O to the path to test if it is working (otherwise, your device will keep failing over). This almost always means that you set the path_checker to tur; this works for all SCSI devices that support the Test Unit Ready command, which most do.
If the device needs a special command to switch paths, then configuring this device for multipath requires a hardware handler kernel module. The current hardware handlers are emc and "rdac. If these are not sufficient for your device, you may not be able to configure the device for multipath.
Table 4.3. Device Attributes
Attribute Description
vendor Specifies the vendor name of the storage device to which the device attributes apply, for example COMPAQ.
product Specifies the product name of the storage device to which the device attributes apply, for example HSV110 (C)COMPAQ.
bl_product Specifies a regular expression used to blacklist devices by vendor/product. Note that for a device to get blacklisted, the vendor, product, and bl_product strings must all match.
path_grouping_policy Specifies the default path grouping policy to apply to unspecified multipaths. Possible values include:
failover = 1 path per priority group
multibus = all valid paths in 1 priority group
group_by_serial = 1 priority group per detected serial number
group_by_prio = 1 priority group per path priority value
group_by_node_name = 1 priority group per target node name
getuid_callout Specifies the default program and arguments to call out to obtain a unique path identifier. An absolute path is required.
prio_callout Specifies the default program and arguments to call out to obtain a path weight. Weights are summed for each path group to determine the next path group to use in case of failure. "none" is a valid value.
path_checker Specifies the default method used to determine the state of the paths. Possible values include readsector0 and tur, emc_clariion, hp_sw, and directio.
path_selector Specifies the default algorithm to use in determining what path to use for the next I/O operation.
failback Specifies path group failback.
A value of 0 or immediate specifies that as soon as there is a path group with a higher priority than the current path group the system switches to that path group.
A numeric value greater than zero specifies deferred failback, expressed in seconds.
A value of manual specifies that failback can happen only with operator intervention.
features The extra features of multipath devices. The only existing feature is queue_if_no_path, which is the same as setting no_path_retry to queue.
hardware_handler Specifies a module that will be used to perform hardware specific actions when switching path groups or handling I/O errors. Possible values include 0, 1 emc, and 1 rdac. The default value is 0.
rr_min_io (RHEL 4.8 and later) Specifies the number of I/O requests to route to a path before switching to the next path in the current path group. The default value is 1000.
rr_weight If set to priorities, then instead of sending rr_min_io requests to a path before calling selector to choose the next path, the number of requests to send is determined by rr_min_io times the path's priority, as determined by the prio_callout program. Currently, there are priority callouts only for devices that use the group_by_prio path grouping policy, which means that all the paths in a path group will always have the same priority.
If set to uniform, all path weights are equal. The default value is uniform.
no_path_retry
A numeric value for this attribute specifies the number of times the system should attempt to use a failed path before disabling queueing.
A value of fail indicates immediate failure, without queuing.
A value of queue indicates that queuing should not stop until the path is fixed.
The default value is (null).
flush_on_last_del (RHEL 4.7 and later) If set to yes, the multipathd daemon will disable queueing when the last path to a device has been deleted. The default value is no.
The following example shows a device entry in the multipath configuration file. 
#	}
#	device {
#		vendor			"COMPAQ  "
#		product			"MSA1000         "
#		path_grouping_policy	multibus
#		path_checker		tur
#		rr_weight		priorities
#	}
#}

Chapter 5. DM-Multipath Administration and Troubleshooting

This chapter will provide information on administering DM-Multipath on a running system. It includes sections on the following topics:
  • Multipath Command Output
  • Multipath Queries with multipath Command
  • Multipath Command Options
  • Multipath Queries with dmsetup Command
  • Troubleshooting with the multipathd Interactive Console
  • Resizing an Online Multipathed Device

5.1. Multipath Command Output

When you create, modify, or list a multipath device, you get a printout of the current device setup. The format is as follows.
For each multipath device: 
 action_if_any: alias (wwid_if_different_from_alias) [size][features][hardware_handler]
For each path group: 
\_ scheduling_policy [path_group_priority_if_known] [path_group_status_if_known]
For each path: 
\_ host:channel:id:lun devnode major:minor [path_status] [dm_status_if_known]
For example, the output of a multipath command might appear as follows: 
mpath1 (3600d0230003228bc000339414edb8101) [size=10 GB][features="0"][hwhandler="0"]
\_ round-robin 0 [prio=1][active]
 \_ 2:0:0:6 sdb 8:16 [active][ready]
\_ round-robin 0 [prio=1][enabled]
 \_ 3:0:0:6 sdc 8:64 [active][ready]
If the path is up and ready for I/O, the status of the path is ready or active. If the path is down, the status is faulty or failed. The path status is updated periodically by the multipathd daemon based on the polling interval defined in the /etc/multipath.conf file.
The dm status is similar to the path status, but from the kernel's point of view. The dm status has two states: failed, which is analogous to faulty, and active which covers all other path states. Occasionally, the path state and the dm state of a device will temporarily not agree.

Note

When a multipath device is being created or modified, the path group status and the dm status are not known. Also, the features are not always correct. When a multipath device is being listed, the path group priority is not known.

5.2. Multipath Queries with multipath Command

You can use the -l and -ll options of the multipath command to display the current multipath configuration. The -l option displays multipath topology gathered from information in sysfs and the device mapper. The -ll option displays the information the -l displays in addition to all other available components of the system.
When displaying the multipath configuration, there are three verbosity levels you can specify with the -v option of the multipath command. Specifying -v0 yields no output. Specifying -v1 outputs the created or updated multipath names only, which you can then feed to other tools such as kpartx. Specifying -v2 prints all detected paths, multipaths, and device maps.
The following example shows the output of a multipath -l command. 
# multipath -l
mpath1 (3600d0230003228bc000339414edb8101)
[size=10 GB][features="0"][hwhandler="0"]
\_ round-robin 0 [prio=1][active]
 \_ 2:0:0:6 sdb 8:16 [active][ready]
\_ round-robin 0 [prio=1][enabled]
 \_ 3:0:0:6 sdc 8:64 [active][ready]

5.3. Multipath Command Options

Table 5.1, “Useful multipath Command Options” describes some options of the multipath command that you may find useful.
Table 5.1. Useful multipath Command Options
Option Description
-l Display the current multipath configuration gathered from sysfs and the device mapper.
-ll Display the current multipath configuration gathered from sysfs, the device mapper, and all other available components on the system.
-f device Remove the named multipath device.
-F Remove all multipath devices.

5.4. Determining Device Mapper Entries with the dmsetup Command

You can use the dmsetup command to find out which device mapper entries match the multipathed devices.
The following command displays all the device mapper devices and their major and minor numbers. The minor numbers determine the name of the dm device. For example, a minor number of 3 corresponds to the multipathed device /dev/dm-3. 
# dmsetup ls
mpath2  (253, 4)
mpath4p1        (253, 12)
mpath5p1        (253, 11)
mpath1  (253, 3)
mpath6p1        (253, 14)
mpath7p1        (253, 13)
mpath0  (253, 2)
mpath7  (253, 9)
mpath6  (253, 8)
VolGroup00-LogVol01     (253, 1)
mpath5  (253, 7)
VolGroup00-LogVol00     (253, 0)
mpath4  (253, 6)
mpath1p1        (253, 10)
mpath3  (253, 5)

5.5. Troubleshooting with the multipathd Interactive Console

The multipathd -k command is an interactive interface to the multipathd daemon. Entering this command brings up an interactive multipath console. After entering this command, you can enter help to get a list of available commands, you can enter a interactive command, or you can enter CTRL-D to quit.
The multipathd interactive console can be used to troubleshoot problems you may be having with your system. For example, the following command sequence displays the multipath configuration, including the defaults, before exiting the console. 
# multipathd -k
> > show config
> > CTRL-D
The following command sequence ensures that multipath has picked up any changes to the multipath.conf, 
# multipathd -k
> > reconfigure
> > CTRL-D
Use the following command sequence to ensure that the path checker is working properly. 
# multipathd -k
> > show paths
> > CTRL-D

5.6. Resizing an Online Multipathed Device (RHEL 4.8 and later)

In systems running RHEL 4.8 and later, is is possible to resize a multipath device while it is online. This allows you to resize the device when it is open, as when a file system is currently mounted.
Use the following procedure to resize an online multipath device.
  1. Resize your physical device.
  2. Resize your paths. For SCSI devices, writing a 1 to the rescan file for the device causes the SCSI driver to rescan. You can use the following command: 
    # echo 1 > /sys/block/device_name/device/rescan
  3. Resize your multipath device by running the multipath command: 
    #  multipath
Your hardware setup may require that you temporarily take the actual storage offline in order to resize your physical device. If you take your storage offline and your multipath device is not set to queue when all paths are down, any I/O activity while your storage is offline will fail. You can work around this by executing the following command before taking your storage offline: 
#  dmsetup suspend --noflush device_name
After you resize your storage and take it back online, you must run the following command before resizing your paths: 
#  dmsetup resume device_name

Appendix A. Revision History

Revision History
Revision 1.0-11.4002013-10-31Rüdiger Landmann
Rebuild with publican 4.0.0
Revision 1.0-112012-07-18Anthony Towns
Rebuild for Publican 3.0
Revision 4.9-1Wed Feb 16 2011Steven Levine
Resolves: #633518
Fixes typo.
Revision 1.0-0Wed Apr 01 2009

Index

A

active/active configuration
definition, Overview of DM-Multipath
illustration, Overview of DM-Multipath
active/passive configuration
definition, Overview of DM-Multipath
illustration, Overview of DM-Multipath
alias parameter , Multipaths Device Configuration Attributes
configuration file, Multipath Device Identifiers

B

bindings_file parameter, Configuration File Defaults
blacklist
configuration file, Configuration File Blacklist
default devices, Blacklisting By Device Name
device name, Blacklisting By Device Name
in configuration file, Setting Up DM-Multipath
WWID, Blacklisting By WWID
bl_product parameter, Configuration File Devices

C

chkconfig command, Setting Up DM-Multipath
configuration file
alias parameter, Multipaths Device Configuration Attributes
bindings_file parameter, Configuration File Defaults
blacklist, Setting Up DM-Multipath, Configuration File Blacklist
bl_product parameter, Configuration File Devices
devnode_blacklist, Configuration File Blacklist
failback parameter, Configuration File Defaults, Multipaths Device Configuration Attributes, Configuration File Devices
features parameter, Configuration File Defaults, Configuration File Devices
flush_on_last_del parameter, Configuration File Defaults
getuid_callout parameter, Configuration File Defaults, Configuration File Devices
gid parameter, Configuration File Defaults
hardware_handler parameter, Configuration File Devices
max_fds parameter, Configuration File Devices
mode parameter, Configuration File Defaults
no_path_retry parameter, Configuration File Defaults, Multipaths Device Configuration Attributes, Configuration File Devices
overview, Configuration File Overview
path_checker parameter, Configuration File Defaults, Configuration File Devices
path_grouping_policy parameter, Configuration File Defaults, Multipaths Device Configuration Attributes, Configuration File Devices
path_selector parameter, Multipaths Device Configuration Attributes, Configuration File Devices
polling-interval parameter, Configuration File Defaults
prio_callout parameter, Configuration File Defaults, Configuration File Devices
product parameter, Configuration File Devices
rr_min_io parameter, Configuration File Defaults, Multipaths Device Configuration Attributes
rr_weight parameter, Configuration File Defaults, Multipaths Device Configuration Attributes, Configuration File Devices
selector parameter, Configuration File Defaults
udev_dir parameter, Configuration File Defaults
uid parameter, Configuration File Defaults
user_friendly_names parameter, Configuration File Defaults
vendor parameter, Configuration File Devices
wwid parameter, Multipaths Device Configuration Attributes
configuring
DM-Multipath, Setting Up DM-Multipath

D

defaults section
multipath.conf file, Configuration File Defaults
dev/mapper directory, Multipath Device Identifiers
dev/mpath directory, Multipath Device Identifiers
device name, Multipath Device Identifiers
device-mapper-multipath package, Setting Up DM-Multipath
devices
adding, Adding Devices to the Multipathing Database , Configuration File Devices
devices section
multipath.conf file, Configuration File Devices
devnode_blacklist
configuration file, Configuration File Blacklist
DM-Multipath
and LVM, Multipath Devices in Logical Volumes
components, DM-Multipath Components
configuration file, The DM-Multipath Configuration File
configuring, Setting Up DM-Multipath
definition, Device Mapper Multipathing
device name, Multipath Device Identifiers
devices, Multipath Devices
failover, Overview of DM-Multipath
overview, Overview of DM-Multipath
redundancy, Overview of DM-Multipath
setup, Setting Up DM-Multipath
setup, overview, DM-Multipath Setup Overview
dm-multipath kernel module , DM-Multipath Components
dm-n devices, Multipath Device Identifiers
dmsetup command, determining device mapper entries, Determining Device Mapper Entries with the dmsetup Command

E

etc/multipath.conf package, Setting Up DM-Multipath

F

failback parameter, Configuration File Defaults, Multipaths Device Configuration Attributes, Configuration File Devices
failover, Overview of DM-Multipath
features parameter, Configuration File Defaults, Configuration File Devices
feedback, Feedback
flush_on_last_del parameter, Configuration File Defaults

G

getuid_callout parameter, Configuration File Defaults, Configuration File Devices
gid parameter, Configuration File Defaults

H

hardware_handler parameter, Configuration File Devices

K

kpartx command , DM-Multipath Components

L

local disks, ignoring, Ignoring Local Disks when Generating Multipath Devices
LVM physical volumes
multipath devices, Multipath Devices in Logical Volumes
lvm.conf file , Multipath Devices in Logical Volumes

M

max_fds parameter, Configuration File Devices
mode parameter, Configuration File Defaults
modprobe command, Setting Up DM-Multipath
multipath command , DM-Multipath Components, Setting Up DM-Multipath
options, Multipath Command Options
output, Multipath Command Output
queries, Multipath Queries with multipath Command
multipath devices, Multipath Devices
logical volumes, Multipath Devices in Logical Volumes
LVM physical volumes, Multipath Devices in Logical Volumes
multipath.conf file, Storage Array Support, The DM-Multipath Configuration File
defaults section, Configuration File Defaults
devices section, Configuration File Devices
multipaths section, Multipaths Device Configuration Attributes
multipath.conf.annotated file, The DM-Multipath Configuration File
multipath.conf.defaults file, Storage Array Support, The DM-Multipath Configuration File
multipathd
command, Troubleshooting with the multipathd Interactive Console
interactive console, Troubleshooting with the multipathd Interactive Console
multipathd daemon , DM-Multipath Components
multipathd start command, Setting Up DM-Multipath
multipaths section
multipath.conf file, Multipaths Device Configuration Attributes

N

no_path_retry parameter, Configuration File Defaults, Multipaths Device Configuration Attributes, Configuration File Devices

P

path_checker parameter, Configuration File Defaults, Configuration File Devices
path_grouping_policy parameter, Configuration File Defaults, Multipaths Device Configuration Attributes, Configuration File Devices
path_selector parameter, Multipaths Device Configuration Attributes, Configuration File Devices
polling_interval parameter, Configuration File Defaults
prio_callout parameter, Configuration File Defaults, Configuration File Devices
product parameter, Configuration File Devices

R

rr_min_io parameter, Configuration File Defaults, Multipaths Device Configuration Attributes
rr_weight parameter, Configuration File Defaults, Multipaths Device Configuration Attributes, Configuration File Devices

S

selector parameter, Configuration File Defaults
setup
DM-Multipath, Setting Up DM-Multipath
storage array support, Storage Array Support
storage arrays
adding, Adding Devices to the Multipathing Database , Configuration File Devices

U

udev_dir parameter, Configuration File Defaults
uid parameter, Configuration File Defaults
user_friendly_names parameter , Multipath Device Identifiers, Configuration File Defaults

V

vendor parameter, Configuration File Devices

W

World Wide Identifier (WWID), Multipath Device Identifiers
wwid parameter, Multipaths Device Configuration Attributes

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