Chapter 2. Maintaining VDO

After deploying a VDO volume, you can perform certain tasks to maintain or optimize it. Some of the following tasks are required for the correct functioning of VDO volumes.

2.1. Prerequisites

2.2. Managing free space on VDO volumes

VDO is a thinly provisioned block storage target. Because of that, you must actively monitor and manage space usage on VDO volumes.

2.2.1. Thin provisioning in VDO

VDO is a thinly provisioned block storage target. The amount of physical space that a VDO volume uses might differ from the size of the volume that is presented to users of the storage.

You can make use of this disparity to save on storage costs. Take care to avoid unexpectedly running out of storage space, if the data written does not achieve the expected rate of optimization.

Whenever the number of logical blocks (virtual storage) exceeds the number of physical blocks (actual storage), it becomes possible for file systems and applications to unexpectedly run out of space. For that reason, storage systems using VDO must provide you with a way of monitoring the size of the free pool on the VDO volume.

You can determine the size of this free pool by using the vdostats utility. The default output of this utility lists information for all running VDO volumes in a format similar to the Linux df utility. For example:

Device              1K-blocks   Used        Available   Use%
/dev/mapper/my-vdo  211812352   105906176   105906176     50%

When the physical storage capacity of a VDO volume is almost full, VDO reports a warning in the system log, similar to the following:

Oct  2 17:13:39 system lvm[13863]: Monitoring VDO pool my-vdo.
Oct  2 17:27:39 system lvm[13863]: WARNING: VDO pool my-vdo is now 80.69% full.
Oct  2 17:28:19 system lvm[13863]: WARNING: VDO pool my-vdo is now 85.25% full.
Oct  2 17:29:39 system lvm[13863]: WARNING: VDO pool my-vdo is now 90.64% full.
Oct  2 17:30:29 system lvm[13863]: WARNING: VDO pool my-vdo is now 96.07% full.
Note

These warning messages appear only when the lvm2-monitor service is running. It is enabled by default.

If the size of free pool drops below a certain level, you can take action by:

  • Deleting data. This reclaims space whenever the deleted data is not duplicated. Deleting data frees the space only after discards are issued.
  • Adding physical storage
  • Deleting LUNs on top of VDO
Important

Monitor physical space on your VDO volumes to prevent out-of-space situations. Running out of physical blocks might result in losing recently written, unacknowledged data on the VDO volume.

2.2.2. Monitoring VDO

This procedure describes how to obtain usage and efficiency information from a VDO volume.

Prerequisites
Procedure
  • Use the vdostats utility to get information about a VDO volume:

    # vdostats --human-readable
    
    Device                   1K-blocks    Used     Available    Use%    Space saving%
    /dev/mapper/node1osd1    926.5G       21.0G    905.5G       2%      73%
    /dev/mapper/node1osd2    926.5G       28.2G    898.3G       3%      64%
Additional resources
  • The vdostats(8) man page.

2.2.3. Reclaiming space for VDO on file systems

This procedure reclaims storage space on a VDO volume that hosts a file system.

VDO cannot reclaim space unless file systems communicate that blocks are free using the DISCARD, TRIM, or UNMAP commands.

Procedure
  • If the file system on your VDO volume supports discard operations, enable them. See Chapter 3, Discarding unused blocks.
  • For file systems that do not use DISCARD, TRIM, or UNMAP, you can manually reclaim free space. Store a file consisting of binary zeros and then delete that file.

2.2.4. Reclaiming space for VDO without a file system

This procedure reclaims storage space on a VDO volume that is used as a block storage target without a file system.

Procedure
  • Use the blkdiscard utility.

    For example, a single VDO volume can be carved up into multiple subvolumes by deploying LVM on top of it. Before deprovisioning a logical volume, use the blkdiscard utility to free the space previously used by that logical volume.

    LVM supports the REQ_DISCARD command and forwards the requests to VDO at the appropriate logical block addresses in order to free the space. If you use other volume managers, they also need to support REQ_DISCARD, or equivalently, UNMAP for SCSI devices or TRIM for ATA devices.

Additional resources
  • The blkdiscard(8) man page

2.2.5. Reclaiming space for VDO on Fibre Channel or Ethernet network

This procedure reclaims storage space on VDO volumes (or portions of volumes) that are provisioned to hosts on a Fibre Channel storage fabric or an Ethernet network using SCSI target frameworks such as LIO or SCST.

Procedure
  • SCSI initiators can use the UNMAP command to free space on thinly provisioned storage targets, but the SCSI target framework needs to be configured to advertise support for this command. This is typically done by enabling thin provisioning on these volumes.

    Verify support for UNMAP on Linux-based SCSI initiators by running the following command:

    # sg_vpd --page=0xb0 /dev/device

    In the output, verify that the Maximum unmap LBA count value is greater than zero.

2.3. Starting or stopping VDO volumes

You can start or stop a given VDO volume, or all VDO volumes, and their associated UDS indexes.

2.3.1. Started and activated VDO volumes

During the system boot, the vdo systemd unit automatically starts all VDO devices that are configured as activated.

The vdo systemd unit is installed and enabled by default when the vdo package is installed. This unit automatically runs the vdo start --all command at system startup to bring up all activated VDO volumes.

You can also create a VDO volume that does not start automatically by adding the --activate=disabled option to the vdo create command.

The starting order

Some systems might place LVM volumes both above VDO volumes and below them. On these systems, it is necessary to start services in the right order:

  1. The lower layer of LVM must start first. In most systems, starting this layer is configured automatically when the LVM package is installed.
  2. The vdo systemd unit must start then.
  3. Finally, additional scripts must run in order to start LVM volumes or other services on top of the running VDO volumes.

How long it takes to stop a volume

Stopping a VDO volume takes time based on the speed of your storage device and the amount of data that the volume needs to write:

  • The volume always writes around 1GiB for every 1GiB of the UDS index.
  • With a sparse UDS index, the volume additionally writes the amount of data equal to the block map cache size plus up to 8MiB per slab.

2.3.2. Starting a VDO volume

This procedure starts a given VDO volume or all VDO volumes on your system.

Procedure
  • To start a given VDO volume, use:

    # vdo start --name=my-vdo
  • To start all VDO volumes, use:

    # vdo start --all
Additional resources
  • The vdo(8) man page

2.3.3. Stopping a VDO volume

This procedure stops a given VDO volume or all VDO volumes on your system.

Procedure
  1. Stop the volume.

    • To stop a given VDO volume, use:

      # vdo stop --name=my-vdo
    • To stop all VDO volumes, use:

      # vdo stop --all
  2. Wait for the volume to finish writing data to the disk.
Additional resources
  • The vdo(8) man page

2.4. Automatically starting VDO volumes at system boot

You can configure VDO volumes so that they start automatically at system boot. You can also disable the automatic start.

2.4.1. Started and activated VDO volumes

During the system boot, the vdo systemd unit automatically starts all VDO devices that are configured as activated.

The vdo systemd unit is installed and enabled by default when the vdo package is installed. This unit automatically runs the vdo start --all command at system startup to bring up all activated VDO volumes.

You can also create a VDO volume that does not start automatically by adding the --activate=disabled option to the vdo create command.

The starting order

Some systems might place LVM volumes both above VDO volumes and below them. On these systems, it is necessary to start services in the right order:

  1. The lower layer of LVM must start first. In most systems, starting this layer is configured automatically when the LVM package is installed.
  2. The vdo systemd unit must start then.
  3. Finally, additional scripts must run in order to start LVM volumes or other services on top of the running VDO volumes.

How long it takes to stop a volume

Stopping a VDO volume takes time based on the speed of your storage device and the amount of data that the volume needs to write:

  • The volume always writes around 1GiB for every 1GiB of the UDS index.
  • With a sparse UDS index, the volume additionally writes the amount of data equal to the block map cache size plus up to 8MiB per slab.

2.4.2. Activating a VDO volume

This procedure activates a VDO volume to enable it to start automatically.

Procedure
  • To activate a specific volume:

    # vdo activate --name=my-vdo
  • To activate all volumes:

    # vdo activate --all
Additional resources
  • The vdo(8) man page

2.4.3. Deactivating a VDO volume

This procedure deactivates a VDO volume to prevent it from starting automatically.

Procedure
  • To deactivate a specific volume:

    # vdo deactivate --name=my-vdo
  • To deactivate all volumes:

    # vdo deactivate --all
Additional resources
  • The vdo(8) man page

2.5. Selecting a VDO write mode

You can configure write mode for a VDO volume, based on what the underlying block device requires. By default, VDO selects write mode automatically.

2.5.1. VDO write modes

VDO supports the following write modes:

sync

When VDO is in sync mode, the layers above it assume that a write command writes data to persistent storage. As a result, it is not necessary for the file system or application, for example, to issue FLUSH or force unit access (FUA) requests to cause the data to become persistent at critical points.

VDO must be set to sync mode only when the underlying storage guarantees that data is written to persistent storage when the write command completes. That is, the storage must either have no volatile write cache, or have a write through cache.

async

When VDO is in async mode, VDO does not guarantee that the data is written to persistent storage when a write command is acknowledged. The file system or application must issue FLUSH or FUA requests to ensure data persistence at critical points in each transaction.

VDO must be set to async mode if the underlying storage does not guarantee that data is written to persistent storage when the write command completes; that is, when the storage has a volatile write back cache.

Warning

When VDO is running in async mode, it is not compliant with Atomicity, Consistency, Isolation, Durability (ACID). When there is an application or a file system that assumes ACID compliance on top of the VDO volume, async mode might cause unexpected data loss.

auto
The auto mode automatically selects sync or async based on the characteristics of each device. This is the default option.

2.5.2. The internal processing of VDO write modes

This section provides details on how the sync and async VDO write modes operate.

If the kvdo module is operating in synchronous mode:

  1. It temporarily writes the data in the request to the allocated block and then acknowledges the request.
  2. Once the acknowledgment is complete, an attempt is made to deduplicate the block by computing a MurmurHash-3 signature of the block data, which is sent to the VDO index.
  3. If the VDO index contains an entry for a block with the same signature, kvdo reads the indicated block and does a byte-by-byte comparison of the two blocks to verify that they are identical.
  4. If they are indeed identical, then kvdo updates its block map so that the logical block points to the corresponding physical block and releases the allocated physical block.
  5. If the VDO index did not contain an entry for the signature of the block being written, or the indicated block does not actually contain the same data, kvdo updates its block map to make the temporary physical block permanent.

If kvdo is operating in asynchronous mode:

  1. Instead of writing the data, it will immediately acknowledge the request.
  2. It will then attempt to deduplicate the block in same manner as described above.
  3. If the block turns out to be a duplicate, kvdo updates its block map and releases the allocated block. Otherwise, it writes the data in the request to the allocated block and updates the block map to make the physical block permanent.

2.5.3. Checking the write mode on a VDO volume

This procedure lists the active write mode on a selected VDO volume.

Procedure
  • Use the following command to see the write mode used by a VDO volume:

    # vdo status --name=my-vdo

    The output lists:

    • The configured write policy, which is the option selected from sync, async, or auto
    • The write policy, which is the particular write mode that VDO applied, that is either sync or async

2.5.4. Checking for a volatile cache

This procedure determines if a block device has a volatile cache or not. You can use the information to choose between the sync and async VDO write modes.

Procedure
  1. Use either of the following methods to determine if a device has a writeback cache:

    • Read the /sys/block/block-device/device/scsi_disk/identifier/cache_type sysfs file. For example:

      $ cat '/sys/block/sda/device/scsi_disk/7:0:0:0/cache_type'
      
      write back
      $ cat '/sys/block/sdb/device/scsi_disk/1:2:0:0/cache_type'
      
      None
    • Alternatively, you can find whether the above mentioned devices have a write cache or not in the kernel boot log:

      sd 7:0:0:0: [sda] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA
      sd 1:2:0:0: [sdb] Write cache: disabled, read cache: disabled, supports DPO and FUA
  2. In the previous examples:

    • Device sda indicates that it has a writeback cache. Use async mode for it.
    • Device sdb indicates that it does not have a writeback cache. Use sync mode for it.

    You should configure VDO to use the sync write mode if the cache_type value is None or write through.

2.5.5. Setting a VDO write mode

This procedure sets a write mode for a VDO volume, either for an existing one or when creating a new volume.

Important

Using an incorrect write mode might result in data loss after a power failure, a system crash, or any unexpected loss of contact with the disk.

Prerequisites
Procedure
  • You can set a write mode either on an existing VDO volume or when creating a new volume:

    • To modify an existing VDO volume, use:

      # vdo changeWritePolicy --writePolicy=sync|async|auto \
                              --name=vdo-name
    • To specify a write mode when creating a VDO volume, add the --writePolicy=sync|async|auto option to the vdo create command.

2.6. Recovering a VDO volume after an unclean shutdown

You can recover a VDO volume after an unclean shutdown to enable it to continue operating. The task is mostly automated. Additionally, you can clean up after a VDO volume was unsuccessfully created because of a failure in the process.

2.6.1. VDO write modes

VDO supports the following write modes:

sync

When VDO is in sync mode, the layers above it assume that a write command writes data to persistent storage. As a result, it is not necessary for the file system or application, for example, to issue FLUSH or force unit access (FUA) requests to cause the data to become persistent at critical points.

VDO must be set to sync mode only when the underlying storage guarantees that data is written to persistent storage when the write command completes. That is, the storage must either have no volatile write cache, or have a write through cache.

async

When VDO is in async mode, VDO does not guarantee that the data is written to persistent storage when a write command is acknowledged. The file system or application must issue FLUSH or FUA requests to ensure data persistence at critical points in each transaction.

VDO must be set to async mode if the underlying storage does not guarantee that data is written to persistent storage when the write command completes; that is, when the storage has a volatile write back cache.

Warning

When VDO is running in async mode, it is not compliant with Atomicity, Consistency, Isolation, Durability (ACID). When there is an application or a file system that assumes ACID compliance on top of the VDO volume, async mode might cause unexpected data loss.

auto
The auto mode automatically selects sync or async based on the characteristics of each device. This is the default option.

2.6.2. VDO volume recovery

When a VDO volume restarts after an unclean shutdown, VDO performs the following actions:

  • Verifies the consistency of the metadata on the volume.
  • Rebuilds a portion of the metadata to repair it if necessary.

Rebuilds are automatic and do not require user intervention.

VDO might rebuild different writes depending on the active write mode:

sync
If VDO was running on synchronous storage and write policy was set to sync, all data written to the volume are fully recovered.
async
If the write policy was async, some writes might not be recovered if they were not made durable. This is done by sending VDO a FLUSH command or a write I/O tagged with the FUA (force unit access) flag. You can accomplish this from user mode by invoking a data integrity operation like fsync, fdatasync, sync, or umount.

In either mode, some writes that were either unacknowledged or not followed by a flush might also be rebuilt.

Automatic and manual recovery

When a VDO volume enters recovering operating mode, VDO automatically rebuilds the unclean VDO volume after the it comes back online. This is called online recovery.

If VDO cannot recover a VDO volume successfully, it places the volume in read-only operating mode that persists across volume restarts. You need to fix the problem manually by forcing a rebuild.

Additional resources

2.6.3. VDO operating modes

This section describes the modes that indicate whether a VDO volume is operating normally or is recovering from an error.

You can display the current operating mode of a VDO volume using the vdostats --verbose device command. See the Operating mode attribute in the output.

normal
This is the default operating mode. VDO volumes are always in normal mode, unless either of the following states forces a different mode. A newly created VDO volume starts in normal mode.
recovering

When a VDO volume does not save all of its metadata before shutting down, it automatically enters recovering mode the next time that it starts up. The typical reasons for entering this mode are sudden power loss or a problem from the underlying storage device.

In recovering mode, VDO is fixing the references counts for each physical block of data on the device. Recovery usually does not take very long. The time depends on how large the VDO volume is, how fast the underlying storage device is, and how many other requests VDO is handling simultaneously. The VDO volume functions normally with the following exceptions:

  • Initially, the amount of space available for write requests on the volume might be limited. As more of the metadata is recovered, more free space becomes available.
  • Data written while the VDO volume is recovering might fail to deduplicate against data written before the crash if that data is in a portion of the volume that has not yet been recovered. VDO can compress data while recovering the volume. You can still read or overwrite compressed blocks.
  • During an online recovery, certain statistics are unavailable: for example, blocks in use and blocks free. These statistics become available when the rebuild is complete.
  • Response times for reads and writes might be slower than usual due to the ongoing recovery work

You can safely shut down the VDO volume in recovering mode. If the recovery does not finish before shutting down, the device enters recovering mode again the next time that it starts up.

The VDO volume automatically exits recovering mode and moves to normal mode when it has fixed all the reference counts. No administrator action is necessary. For details, see Section 2.6.4, “Recovering a VDO volume online”.

read-only

When a VDO volume encounters a fatal internal error, it enters read-only mode. Events that might cause read-only mode include metadata corruption or the backing storage device becoming read-only. This mode is an error state.

In read-only mode, data reads work normally but data writes always fail. The VDO volume stays in read-only mode until an administrator fixes the problem.

You can safely shut down a VDO volume in read-only mode. The mode usually persists after the VDO volume is restarted. In rare cases, the VDO volume is not able to record the read-only state to the backing storage device. In these cases, VDO attempts to do a recovery instead.

Once a volume is in read-only mode, there is no guarantee that data on the volume has not been lost or corrupted. In such cases, Red Hat recommends copying the data out of the read-only volume and possibly restoring the volume from backup.

If the risk of data corruption is acceptable, it is possible to force an offline rebuild of the VDO volume metadata so the volume can be brought back online and made available. The integrity of the rebuilt data cannot be guaranteed. For details, see Section 2.6.5, “Forcing an offline rebuild of a VDO volume metadata”.

2.6.4. Recovering a VDO volume online

This procedure performs an online recovery on a VDO volume to recover metadata after an unclean shutdown.

Procedure
  1. If the VDO volume is not already started, start it:

    # vdo start --name=my-vdo

    No additional steps are necessary. The recovery runs in the background.

  2. If you rely on volume statistics like blocks in use and blocks free, wait until they are available.

2.6.5. Forcing an offline rebuild of a VDO volume metadata

This procedure performs a forced offline rebuild of a VDO volume metadata to recover after an unclean shutdown.

Warning

This procedure might cause data loss on the volume.

Prerequisites
  • The VDO volume is started.
Procedure
  1. Check if the volume is in read-only mode. See the operating mode attribute in the command output:

    # vdo status --name=my-vdo

    If the volume is not in read-only mode, it is not necessary to force an offline rebuild. Perform an online recovery as described in Section 2.6.4, “Recovering a VDO volume online”.

  2. Stop the volume if it is running:

    # vdo stop --name=my-vdo
  3. Restart the volume with the --forceRebuild option:

    # vdo start --name=my-vdo --forceRebuild

2.6.6. Removing an unsuccessfully created VDO volume

This procedure cleans up a VDO volume in an intermediate state. A volume is left in an intermediate state if a failure occurs when creating the volume. This might happen when, for example:

  • The system crashes
  • Power fails
  • The administrator interrupts a running vdo create command
Procedure
  • To clean up, remove the unsuccessfully created volume with the --force option:

    # vdo remove --force --name=my-vdo

    The --force option is required because the administrator might have caused a conflict by changing the system configuration since the volume was unsuccessfully created.

    Without the --force option, the vdo remove command fails with the following message:

    [...]
    A previous operation failed.
    Recovery from the failure either failed or was interrupted.
    Add '--force' to 'remove' to perform the following cleanup.
    Steps to clean up VDO my-vdo:
    umount -f /dev/mapper/my-vdo
    udevadm settle
    dmsetup remove my-vdo
    vdo: ERROR - VDO volume my-vdo previous operation (create) is incomplete

2.7. Optimizing the UDS index

You can configure certain settings of the UDS index to optimize it on your system.

Important

You cannot change the properties of the UDS index after creating the VDO volume.

2.7.1. The UDS index

VDO uses a high-performance deduplication index called UDS to detect duplicate blocks of data as they are being stored.

The UDS index provides the foundation of the VDO product. For each new piece of data, it quickly determines if that piece is identical to any previously stored piece of data. If the index finds match, the storage system can then internally reference the existing item to avoid storing the same information more than once.

The UDS index runs inside the kernel as the uds kernel module.

The deduplication window is the number of previously written blocks that the index remembers. The size of the deduplication window is configurable. For a given window size, the index requires a specific amount of RAM and a specific amount of disk space. The size of the window is usually determined by specifying the size of the index memory using the --indexMem=size option. VDO then determines the amount of disk space to use automatically.

The UDS index consists of two parts:

  • A compact representation is used in memory that contains at most one entry per unique block.
  • An on-disk component that records the associated block names presented to the index as they occur, in order.

UDS uses an average of 4 bytes per entry in memory, including cache.

The on-disk component maintains a bounded history of data passed to UDS. UDS provides deduplication advice for data that falls within this deduplication window, containing the names of the most recently seen blocks. The deduplication window allows UDS to index data as efficiently as possible while limiting the amount of memory required to index large data repositories. Despite the bounded nature of the deduplication window, most datasets which have high levels of deduplication also exhibit a high degree of temporal locality — in other words, most deduplication occurs among sets of blocks that were written at about the same time. Furthermore, in general, data being written is more likely to duplicate data that was recently written than data that was written a long time ago. Therefore, for a given workload over a given time interval, deduplication rates will often be the same whether UDS indexes only the most recent data or all the data.

Because duplicate data tends to exhibit temporal locality, it is rarely necessary to index every block in the storage system. Were this not so, the cost of index memory would outstrip the savings of reduced storage costs from deduplication. Index size requirements are more closely related to the rate of data ingestion. For example, consider a storage system with 100 TB of total capacity but with an ingestion rate of 1 TB per week. With a deduplication window of 4 TB, UDS can detect most redundancy among the data written within the last month.

2.8. Enabling or disabling deduplication in VDO

In some instances, you might want to temporarily disable deduplication of data being written to a VDO volume while still retaining the ability to read to and write from the volume. Disabling deduplication prevents subsequent writes from being deduplicated, but the data that was already deduplicated remains so.

2.8.1. Deduplication in VDO

Deduplication is a technique for reducing the consumption of storage resources by eliminating multiple copies of duplicate blocks.

Instead of writing the same data more than once, VDO detects each duplicate block and records it as a reference to the original block. VDO maintains a mapping from logical block addresses, which are used by the storage layer above VDO, to physical block addresses, which are used by the storage layer under VDO.

After deduplication, multiple logical block addresses can be mapped to the same physical block address. These are called shared blocks. Block sharing is invisible to users of the storage, who read and write blocks as they would if VDO were not present.

When a shared block is overwritten, VDO allocates a new physical block for storing the new block data to ensure that other logical block addresses that are mapped to the shared physical block are not modified.

2.8.2. Enabling deduplication on a VDO volume

This procedure restarts the associated UDS index and informs the VDO volume that deduplication is active again.

Note

Deduplication is enabled by default.

Procedure
  • To restart deduplication on a VDO volume, use the following command:

    # vdo enableDeduplication --name=my-vdo

2.8.3. Disabling deduplication on a VDO volume

This procedure stops the associated UDS index and informs the VDO volume that deduplication is no longer active.

Procedure
  • To stop deduplication on a VDO volume, use the following command:

    # vdo disableDeduplication --name=my-vdo
  • You can also disable deduplication when creating a new VDO volume by adding the --deduplication=disabled option to the vdo create command.

2.9. Enabling or disabling compression in VDO

VDO provides data compression. You can disable it to maximize performance or to speed processing of data that is unlikely to compress, or re-enable it to increase space savings.

2.9.1. Compression in VDO

In addition to block-level deduplication, VDO also provides inline block-level compression using the HIOPS Compression™ technology.

VDO volume compression is on by default.

While deduplication is the optimal solution for virtual machine environments and backup applications, compression works very well with structured and unstructured file formats that do not typically exhibit block-level redundancy, such as log files and databases.

Compression operates on blocks that have not been identified as duplicates. When VDO sees unique data for the first time, it compresses the data. Subsequent copies of data that have already been stored are deduplicated without requiring an additional compression step.

The compression feature is based on a parallelized packaging algorithm that enables it to handle many compression operations at once. After first storing the block and responding to the requestor, a best-fit packing algorithm finds multiple blocks that, when compressed, can fit into a single physical block. After it is determined that a particular physical block is unlikely to hold additional compressed blocks, it is written to storage and the uncompressed blocks are freed and reused.

By performing the compression and packaging operations after having already responded to the requestor, using compression imposes a minimal latency penalty.

2.9.2. Enabling compression on a VDO volume

This procedure enables compression on a VDO volume to increase space savings.

Note

Compression is enabled by default.

Procedure
  • To start it again, use the following command:

    # vdo enableCompression --name=my-vdo

2.9.3. Disabling compression on a VDO volume

This procedure stops compression on a VDO volume to maximize performance or to speed processing of data that is unlikely to compress.

Procedure
  • To stop compression on an existing VDO volume, use the following command:

    # vdo disableCompression --name=my-vdo
  • Alternatively, you can disable compression by adding the --compression=disabled option to the vdo create command when creating a new volume.

2.10. Increasing the size of a VDO volume

You can increase the physical size of a VDO volume to utilize more underlying storage capacity, or the logical size to provide more capacity on the volume.

2.10.1. Thin provisioning in VDO

VDO is a thinly provisioned block storage target. The amount of physical space that a VDO volume uses might differ from the size of the volume that is presented to users of the storage.

You can make use of this disparity to save on storage costs. Take care to avoid unexpectedly running out of storage space, if the data written does not achieve the expected rate of optimization.

Whenever the number of logical blocks (virtual storage) exceeds the number of physical blocks (actual storage), it becomes possible for file systems and applications to unexpectedly run out of space. For that reason, storage systems using VDO must provide you with a way of monitoring the size of the free pool on the VDO volume.

You can determine the size of this free pool by using the vdostats utility. The default output of this utility lists information for all running VDO volumes in a format similar to the Linux df utility. For example:

Device              1K-blocks   Used        Available   Use%
/dev/mapper/my-vdo  211812352   105906176   105906176     50%

When the physical storage capacity of a VDO volume is almost full, VDO reports a warning in the system log, similar to the following:

Oct  2 17:13:39 system lvm[13863]: Monitoring VDO pool my-vdo.
Oct  2 17:27:39 system lvm[13863]: WARNING: VDO pool my-vdo is now 80.69% full.
Oct  2 17:28:19 system lvm[13863]: WARNING: VDO pool my-vdo is now 85.25% full.
Oct  2 17:29:39 system lvm[13863]: WARNING: VDO pool my-vdo is now 90.64% full.
Oct  2 17:30:29 system lvm[13863]: WARNING: VDO pool my-vdo is now 96.07% full.
Note

These warning messages appear only when the lvm2-monitor service is running. It is enabled by default.

If the size of free pool drops below a certain level, you can take action by:

  • Deleting data. This reclaims space whenever the deleted data is not duplicated. Deleting data frees the space only after discards are issued.
  • Adding physical storage
  • Deleting LUNs on top of VDO
Important

Monitor physical space on your VDO volumes to prevent out-of-space situations. Running out of physical blocks might result in losing recently written, unacknowledged data on the VDO volume.

2.10.2. Increasing the logical size of a VDO volume

This procedure increases the logical size of a given VDO volume. It enables you to initially create VDO volumes that have a logical size small enough to be safe from running out of space. After some period of time, you can evaluate the actual rate of data reduction, and if sufficient, you can grow the logical size of the VDO volume to take advantage of the space savings.

It is not possible to decrease the logical size of a VDO volume.

Procedure
  • To grow the logical size, use:

    # vdo growLogical --name=my-vdo \
                      --vdoLogicalSize=new-logical-size

    When the logical size increases, VDO informs any devices or file systems on top of the volume of the new size.

2.10.3. Increasing the physical size of a VDO volume

This procedure increases the amount of physical storage available to a VDO volume.

It is not possible to shrink a VDO volume in this way.

Prerequisites
  • The underlying block device has a larger capacity than the current physical size of the VDO volume.

    If it does not, you can attempt to increase the size of the device. The exact procedure depends on the type of the device. For example, to resize an MBR or GPT partition, see the Resizing a partition section in the Managing storage devices guide.

Procedure
  • Add the new physical storage space to the VDO volume:

    # vdo growPhysical --name=my-vdo

2.11. Removing VDO volumes

You can remove an existing VDO volume on your system.

2.11.1. Removing a working VDO volume

This procedure removes a VDO volume and its associated UDS index.

Procedure
  1. Unmount the file systems and stop the applications that are using the storage on the VDO volume.
  2. To remove the VDO volume from your system, use:

    # vdo remove --name=my-vdo

2.11.2. Removing an unsuccessfully created VDO volume

This procedure cleans up a VDO volume in an intermediate state. A volume is left in an intermediate state if a failure occurs when creating the volume. This might happen when, for example:

  • The system crashes
  • Power fails
  • The administrator interrupts a running vdo create command
Procedure
  • To clean up, remove the unsuccessfully created volume with the --force option:

    # vdo remove --force --name=my-vdo

    The --force option is required because the administrator might have caused a conflict by changing the system configuration since the volume was unsuccessfully created.

    Without the --force option, the vdo remove command fails with the following message:

    [...]
    A previous operation failed.
    Recovery from the failure either failed or was interrupted.
    Add '--force' to 'remove' to perform the following cleanup.
    Steps to clean up VDO my-vdo:
    umount -f /dev/mapper/my-vdo
    udevadm settle
    dmsetup remove my-vdo
    vdo: ERROR - VDO volume my-vdo previous operation (create) is incomplete