3.11. Managing thin-provisioned LVM-VDO volumes

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>

      注記

      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.

      注記

      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.