Chapter 3. Creating a deduplicated and compressed logical volume

Procedure

1. Create the LVM-VDO:

   # lvcreate --type vdo \
              --name vdo1 \
              --size 1T \
              --virtualsize 10T \
              vg-name

   Replace 1T with the physical size. If the physical size is larger than 16 TiB, add the following option to increase the slab size on the volume to 32 GiB:

   --vdosettings 'vdo_slab_size_mb=32768'

   If you use the default slab size of 2 GiB on a physical size larger than 16 TiB, the lvcreate command fails with the following error:

   ERROR - vdoformat: formatVDO failed on '/dev/device': VDO Status: Exceeds maximum number of slabs supported

   Replace 10T with the logical storage that the LVM-VDO will present.

   Replace vg-name with the name of an existing LVM volume group where you want to place the LVM-VDO.

   Important

   If creating the LVM-VDO volume fails, use lvremove vg-name to remove the volume. Depending on the reason of the failure, you might also need to add two force options (-ff).

2. Create a file system on the LVM-VDO:

   • For the XFS file system:

     # mkfs.xfs -K /dev/vg-name/vdo-name

   • For the ext4 file system:

     # mkfs.ext4 -E nodiscard /dev/vg-name/vdo-name

3. Mount the file system on the LVM-VDO volume:

   • To mount the file system persistently, add the following line to the /etc/fstab file:

     /dev/vg-name/vdo-name mount-point <file-system-type> defaults 0 0

     Replace <file-system-type> with your file system, such as xfs or ext4.

   • To mount the file system manually, use the mount command:

     # mount /dev/vg-name/vdo-name mount-point

Procedure

1. Create a playbook file, for example ~/playbook.yml, with the following content:

   ---
   - name: Manage local storage
     hosts: managed-node-01.example.com
     tasks:
       - name: Create LVM-VDO volume under volume group 'myvg'
         ansible.builtin.include_role:
           name: rhel-system-roles.storage
         vars:
           storage_pools:
             - name: myvg
               disks:
                 - /dev/sdb
               volumes:
                 - name: mylv1
                   compression: true
                   deduplication: true
                   vdo_pool_size: 10 GiB
                   size: 30 GiB
                   mount_point: /mnt/app/shared

   The settings specified in the example playbook include the following:

   vdo_pool_size: <size>

   The actual size that the volume takes on the device. You can specify the size in human-readable format, such as 10 GiB. If you do not specify a unit, it defaults to bytes.

   size: <size>

   The virtual size of VDO volume.

   For details about all variables used in the playbook, see the /usr/share/ansible/roles/rhel-system-roles.storage/README.md file on the control node.

2. Validate the playbook syntax:

   $ ansible-playbook --syntax-check ~/playbook.yml

   Note that this command only validates the syntax and does not protect against a wrong but valid configuration.

3. Run the playbook:

   $ ansible-playbook ~/playbook.yml

Procedure

1. Log in to the RHEL 8 web console.

   For details, see Logging in to the web console.

2. In the panel, click Storage.
3. Click the menu button [⋮] for the LVM2 group in which you want to create an LVM-VDO volume and click Create new logical volume.
4. In the Name field, enter a name for your LVM-VDO volume without spaces.
5. From the Purpose drop-down list, select VDO filesystem volume.
6. In the Size slider, set up the physical size of your LVM-VDO volume.

7. In the Logical size slider, set up the size of the LVM-VDO volume. You can extend it more than ten times, but consider for what purpose you are creating the LVM-VDO volume:

   • For active VMs or container storage, use logical size that is ten times the physical size of the volume.
   • For object storage, use logical size that is three times the physical size of the volume.
8. In the Options list, select Compression and Deduplication.
9. Click Create.

Procedure

1. Log in to the RHEL 8 web console.

   For details, see Logging in to the web console.

2. In the panel, click Storage.
3. Click the LVM2 volume group that has the LVM-VDO volume you want to format and click the menu button [⋮] for the LVM-VDO volume.
4. In the drop-down menu, click Format.
5. In the Name field, enter the logical volume name.
6. In the Mount Point field, enter the mount path.
7. From the Type drop-down list, select a file system.
8. Optional: Select Overwrite existing data with zeros, if the disk includes any sensitive data and you want to rewrite them. Otherwise the web console rewrites only the disk header.
9. From the Encryption drop-down list, select the type of encryption.
10. From the At boot drop-down list, select when you want to mount the volume.

11. In the Mount options list, choose the appropriate settings:

    1. Select the Mount read only checkbox if you want to mount the volume as a read-only logical volume.
    2. Select the Custom mount options checkbox and add the mount options if you want to change the default mount option.

12. Format the LVM-VDO volume:

    1. If you want to format and mount the LVM-VDO volume, click Format and mount.
    2. If you want to only format the partition, click Format only.

Procedure

1. Log in to the RHEL 8 web console.

   For details, see Logging in to the web console.

2. In the panel, click Storage.
3. Click your LVM-VDO volume in the VDO Devices box.
4. In the LVM-VDO volume details, click Grow.
5. In the Grow logical size of VDO dialog box, extend the logical size of the LVM-VDO volume.
6. Click Grow.

Procedure

1. Check the compression status for your logical volumes:

   # lvs -o+vdo_compression,vdo_compression_state
     LV         VG        Attr         LSize   Pool   Origin Data%  Meta%  Move Log Cpy%Sync Convert VDOCompression VDOCompressionState
     vdo_name vg_name vwi-a-v---   1.00t vpool0        0.00                                           enabled online
     vpool0   vg_name dwi------- <15.00g                 20.03                                          enabled online

2. Disable the compression for VDOPoolLV:

   # lvchange --compression n vg-name/vdopoolname

   If you want to enable the compression, use the y option instead of n.

Procedure

1. Check the deduplication status for your logical volumes:

   # lvs -o+vdo_deduplication,vdo_index_state
     LV         VG        Attr         LSize   Pool   Origin Data%  Meta%  Move Log Cpy%Sync Convert VDODeduplication VDOIndexState
     vdo_name vg_name vwi-a-v---   1.00t vpool0        0.00                                             enabled  online
     vpool0   vg_name dwi------- <15.00g                 20.03                                            enabled  online

2. Disable the deduplication for VDOPoolLV:

   # lvchange --deduplication n vg-name/vdopoolname

   If you want to enable the deduplication, use the y option instead of n.

Prerequisites

1. Install the VDO software. For more information, see Installing VDO.
2. An LVM volume group with free storage capacity exists on your system.
3. A thin-provisioned LVM-VDO volume using the lvcreate --type vdo --name myvdo myvg -l percentage-of-free-space-in-vg --virtualsize virtual-size-of-vdo command. For more information, see Creating and mounting an LVM-VDO volume.

Procedure

1. Determine the optimal logical size for a thin-provisioned LVM-VDO volume

   # vdostats myvg-vpool0-vpool
   
   Device               1K-blocks Used     Available  Use% Space saving%
   myvg-vpool0-vpool   104856576  29664088 75192488   28%   69%

   To calculate the space savings ratio, use the following formula:

   Savings ratio = 1 / (1 - Space saving%)

   In this example,

   • there is approximately a 3.22:1 space savings ratio on a data set of about 80 GB.
   • Multiplying the data set size by the ratio would yield a potential logical size of 256 GB if more data with the same space savings were written to the LVM-VDO volume.
   • Adjusting this number downward to 200 GB yields a logical size with a safe margin of free physical space, given the same space savings ratio.

2. Monitor the free physical space in an LVM-VDO volume:

   # vdostats myvg-vpool0-vpool

   This command can be executed periodically to provide monitoring of the used and free physical space of the LVM-VDO volume.

3. Optional: View the warnings on physical space usage on an LVM-VDO volume by using the available /usr/share/doc/vdo/examples/monitor/monitor_check_vdostats_physicalSpace.pl script:

   # /usr/share/doc/vdo/examples/monitor/monitor_check_vdostats_physicalSpace.pl myvg-vpool0-vpool

4. When creating an LVM-VDO volume, the dmeventd monitoring service monitors the usage of physical space in an LVM-VDO volume. This is enabled by default when an LVM-VDO volume is created or started.

   Use the journalctl command to view the output of dmeventd in the logs while monitoring an LVM-VDO volume:

   lvm[8331]: Monitoring VDO pool myvg-vpool0-vpool.
   ...
   
   lvm[8331]: WARNING: VDO pool myvg-vpool0-vpool is now 84.63% full.
   lvm[8331]: WARNING: VDO pool myvg-vpool0-vpool is now 91.01% full.
   lvm[8331]: WARNING: VDO pool myvg-vpool0-vpool is now 97.34% full.

5. Remediate LVM-VDO volumes that are almost out of available physical space. When it is possible to add a physical space to an LVM-VDO volume, but the volume space is full before it can be grown, it may be necessary to temporarily stop I/O to the volume.

   To temporarily stop I/O to the volume, execute the following steps, where LVM-VDO volume myvdo contains a file system mounted on the /users/homeDir path:

   1. Freeze the file system:

      # xfs_freeze -f /users/homeDir
      
      # vgextend myvg /dev/vdc2
      
      # lvextend -L new-size myvg/vpool0
      
      # xfs_freeze -u /users/homeDir

   2. Unmount the file system:

      # umount /users/homeDir
      
      # vgextend myvg /dev/vdc2
      
      # lvextend -L new-size myvg/vpool0
      
      # mount -o discard /dev/myvg/myvdo /users/homeDir

      Note

      Unmounting or freezing a file system with cached data will incur a write of the cached data, which might fill the physical space of the LVM-VDO volume. Consider the maximum amount of cached file system data when setting a monitoring threshold for free physical space on an LVM-VDO volume.

6. Blocks that are no longer used by a file system can be cleaned up by using the fstrim utility. Executing fstrim against a mounted file system on top of a VDO volume can result in increased free physical space for that volume. The fstrim utility will send discards to the LVM-VDO volume, which are then used to remove references to the previously used blocks. If any of those blocks are single-referenced, the physical space will be available to use.

   1. Check VDO stats to see what the current amount of free space is:

      # vdostats --human-readable myvg-vpool0-vpool
      
       Device             Size  Used  Available Use%  Space saving%
      myvg-vpool0-vpool  100.0G 95.0G 5.0G      95%   73%

   2. Discard unused blocks:

      # fstrim /users/homeDir

   3. View the free physical space of the LVM-VDO volume:

      # vdostats --human-readable myvg-vpool0-vpool
      
       Device             Size    Used   Available Use%  Space saving%
      myvg-vpool0-vpool  100.0G   30.0G  70.0G     30%    43%

      In this example, after executing fstrim on the file system, the discards were able to return 65 G of physical space to use in the LVM-VDO volume.

      Note

      Discarding volumes with lower levels of deduplication and compression will have a possibility of reclaiming physical space than discarding volumes with higher levels of deduplication and compression. A volume that has high levels of deduplication and compression can potentially require a more extensive cleanup to reclaim physical space than just simply discarding already unused blocks.