Chapter 3. Creating a deduplicated and compressed logical volume

Procedure

1. Pick a name for your VDO LV, such as vdo1. You must use a different name and device for each VDO LV on the system.

   In all the following steps, replace vdo-name with the name.

2. Create the VDO LV:

   # lvcreate --type vdo \
              --name vdo-name
              --size physical-size
              --virtualsize logical-size \
              vg-name

   • Replace vg-name with the name of an existing LVM volume group where you want to place the VDO LV.
   • Replace logical-size with the amount of logical storage that the VDO LV will present.

   • If the physical size is larger than 16TiB, add the following option to increase the slab size on the volume to 32GiB:

     --config 'allocation/vdo_slab_size_mb=32768'

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

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

     Example 3.1. Creating a VDO LV for container storage

     For example, to create a VDO LV for container storage on a 1TB VDO pool LV, you can use:

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

     Important

     If a failure occurs when creating the VDO volume, remove the volume to clean up.

3. Create a file system on the VDO LV:

   • 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

Procedure

1. Log in to the RHEL 9 web console.

   For details, see Logging in to the web console.

2. Click Storage.

3. Click the menu button, ⋮, next to the LVM2 group in which you want to create a VDO volume.

   

4. Select VDO filesystem volume in the drop-down menu next to the Purpose field.
5. In the Name field, enter a name of the VDO volume without spaces.

6. In the Logical Size bar, set up the size of the VDO volume. You can extend it more than ten times, but consider for what purpose you are creating the 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.

   For details, see Deploying VDO.

7. Select the Compression option. This option can efficiently reduce various file formats.

   For details, see Changing the compression settings on an LVM-VDO volume.

8. Select the Deduplication option.

   This option reduces the consumption of storage resources by eliminating multiple copies of duplicate blocks. For details, see Changing the deduplication settings on an LVM-VDO volume.

   

Procedure

1. Log in to the RHEL 9 web console.

   For details, see Logging in to the web console.

2. Click Storage.
3. Click the LVM2 volume group containing the VDO volume you want to format.
4. Click the menu button, ⋮, at the end of the line with the VDO volume you want to format.

5. Click Format.

   

6. In the Name field, enter the logical volume name.
7. In the Mount Point field, add the mount path.
8. By default, the web console rewrites only the disk header after you finish this dialog. The advantage of this option is the speed of formatting. If you check the Overwrite existing data with zeros option, the web console rewrites the whole disk with zeros. This option is slower because the program has to go through the whole disk. Use this option if the disk includes any sensitive data and you want to rewrite them.

9. In the Type drop-down menu, select a file system:

   • The default option, the XFS file system, supports large logical volumes, switching physical drives online without outage, and growing.

     XFS does not support shrinking volumes. Therefore, you cannot reduce the size of a volume formatted with XFS.

   • The ext4 file system supports logical volumes, switching physical drives online without outage, growing, and shrinking.

   You can also select a version with the LUKS (Linux Unified Key Setup) encryption, which allows you to encrypt the volume with a passphrase.

10. In the At boot drop-down menu, select when you want to mount the volume.

11. Click Format and mount or Format only.

    Formatting can take several minutes depending on the used formatting options and the volume size.

    

Procedure

1. Log in to the RHEL 9 web console.

   For details, see Logging in to the web console.

2. Click Storage.
3. Click your VDO volume in the VDO Devices box.
4. In the VDO volume details, click the Grow button.
5. In the Grow logical size of VDO dialog box, extend the logical size of the 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 VDO volume using the lvcreate --type vdo --name myvdo myvg -L logical-size-of-pool --virtualsize virtual-size-of-vdo command. For more information, see Creating an LVM-VDO volume.

Procedure

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

   # vdostats myvg-vpool0-vpool

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

3. Optional: View the warnings on physical space usage on a 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 a VDO volume, the dmeventd monitoring service monitors the usage of physical space in a VDO volume. This is enabled by default when a VDO volume is created or started.

   Use the journalctl command to view the output of dmeventd in the logs while monitoring a 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 VDO volumes that are almost out of available physical space. When it is possible to add a physical space to a 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 VDO volume myvdo contains a file system mounted on the /users/homeDir path:

   1. Freeze the filesystem:

      # xfs_freeze -f /users/homeDir
      
      # vgextend myvg /dev/vdc2
      
      # lvextend -l new_size myvg/vpool0-name
      
      # xfs_freeze -u /users/homeDir

   2. Unmount the filesystem:

      # umount /users/homeDir
      
      # vgextend myvg /dev/vdc2
      
      # lvextend -l new_size myvg/vpool0-name
      
      # mount -o discard /dev/myvg/myvdo /users/homeDir

      Note

      Unmounting or freezing a filesystem with cached data will incur a write of the cached data, which may fill the physical space of the VDO volume. Consider the maximum amount of cached filesystem data when setting a monitoring threshold for free physical space on a 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 may result in increased free physical space for that volume. The fstrim utility will send discards to the 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 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 65G of physical space to use in the 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 may potentially require a more extensive cleanup to reclaim physical space than just simply discarding already unused blocks.