Chapter 10. Snapshot of logical volumes

Using the LVM snapshot feature, you can create virtual images of a volume, for example, /dev/sda, at a particular instant without causing a service interruption.

10.1. Overview of snapshot volumes

When you modify the original volume (the origin) after you take a snapshot, the snapshot feature makes a copy of the modified data area as it was prior to the change so that it can reconstruct the state of the volume. When you create a snapshot, full read and write access to the origin stays possible.

Since a snapshot copies only the data areas that change after the snapshot is created, the snapshot feature requires a minimal amount of storage. For example, with a rarely updated origin, 3-5 % of the origin’s capacity is sufficient to maintain the snapshot. It does not provide a substitute for a backup procedure. Snapshot copies are virtual copies and are not an actual media backup.

The size of the snapshot controls the amount of space set aside for storing the changes to the origin volume. For example, if you create a snapshot and then completely overwrite the origin, the snapshot should be at least as big as the origin volume to hold the changes. You should regularly monitor the size of the snapshot. For example, a short-lived snapshot of a read-mostly volume, such as /usr, would need less space than a long-lived snapshot of a volume because it contains many writes, such as /home.

If a snapshot is full, the snapshot becomes invalid because it can no longer track changes on the origin volume. But you can configure LVM to automatically extend a snapshot whenever its usage exceeds the snapshot_autoextend_threshold value to avoid snapshot becoming invalid. Snapshots are fully resizable and you can perform the following operations:

  • If you have the storage capacity, you can increase the size of the snapshot volume to prevent it from getting dropped.
  • If the snapshot volume is larger than you need, you can reduce the size of the volume to free up space that is needed by other logical volumes.

The snapshot volume provide the following benefits:

  • Most typically, you take a snapshot when you need to perform a backup on a logical volume without halting the live system that is continuously updating the data.
  • You can execute the fsck command on a snapshot file system to check the file system integrity and determine if the original file system requires file system repair.
  • Since the snapshot is read/write, you can test applications against production data by taking a snapshot and running tests against the snapshot without touching the real data.
  • You can create LVM volumes for use with Red Hat Virtualization. You can use LVM snapshots to create snapshots of virtual guest images. These snapshots can provide a convenient way to modify existing guests or create new guests with minimal additional storage.

10.2. Creating a snapshot of the original volume

Use the lvcreate command to create a snapshot of the original volume (the origin). A snapshot of a volume is writable. By default, a snapshot volume is activated with the origin during normal activation commands as compared to the thinly-provisioned snapshots. LVM does not support creating a snapshot volume that is larger than the sum of the origin volume’s size and the required metadata size for the volume. If you specify a snapshot volume that is larger than this, LVM creates a snapshot volume that is required for the size of the origin.

Note

The nodes in a cluster do not support LVM snapshots. You cannot create a snapshot volume in a shared volume group. However, if you need to create a consistent backup of data on a shared logical volume you can activate the volume exclusively and then create the snapshot.

The following procedure creates an origin logical volume named origin and a snapshot volume of this original volume named snap.

Prerequisites

Procedure

  1. Create a logical volume named origin from the volume group vg001: 

    # lvcreate -L 1G -n origin vg001
Logical volume "origin" created.

  2. Create a snapshot logical volume named snap of /dev/vg001/origin that is 100 MB in size: 

    # lvcreate --size 100M --name snap --snapshot /dev/vg001/origin
  Logical volume "snap" created.

    You can also use the -L argument instead of using --size, -n instead of using --name, and -s instead of using --snapshot to create a snapshot.

    If the original logical volume contains a file system, you can mount the snapshot logical volume on an arbitrary directory in to access the contents of the file system to run a backup while the original file system continues to get updated.

  3. Display the origin volume and the current percentage of the snapshot volume being used: 

    # lvs -a -o +devices
  LV      VG    Attr       LSize  Pool Origin Data% Meta% Move Log Cpy%Sync Convert Devices
 origin vg001  owi-a-s---  1.00g                                                  /dev/sde1(0)
  snap vg001  swi-a-s--- 100.00m     origin 0.00                                 /dev/sde1(256)

    You can also display the status of logical volume /dev/vg001/origin with all the snapshot logical volumes and their status, such as active or inactive by using the lvdisplay /dev/vg001/origin command.

    Warning

    Space in the snapshot LV is consumed after the origin LV is written to. The lvs command reports the current snapshot space usage in the Data% data_percent field value. If the snapshot space reaches 100%, the snapshot becomes invalid and unusable.

    An invalid snapshot is reported with I in the fifth position of the Attr column, or the lv_snapshot_invalid reporting field in lvs. You can remove the invalid snapshot by using the lvremove command.

  4. Optional: Extend the snapshot before its space becomes 100% full and becomes invalid by using any one of the following options:

    • Configure LVM to automatically extend the snapshot by using the following parameters in the /etc/lvm.conf file:

      snapshot_autoextend_threshold
      Extends the snapshot after its usage exceeds the value set for this parameter. By default, it is set to 100, which disables automatic extension. The minimum value of this parameter is 50.
      snapshot_autoextend_percent
      Adds an additional space to the snapshot, which is the percent of its current size. By default, it is set to 20.

      In the following example, after setting the following parameters, the created 1G snapshot extends to 1.2G when its usage exceeds 700M:

      Example 10.1. Automatically extend the snapshot

      # vi /etc/lvm.conf
snapshot_autoextend_threshold = 70
snapshot_autoextend_percent = 20
      Note

      This feature requires unallocated space in the volume group. An automatic extension of a snapshot does not increase the size of a snapshot volume beyond the maximum calculated size that is necessary for the snapshot. Once a snapshot has grown large enough to cover the origin, it is no longer monitored for automatic extension.

    • Extend this snapshot manually by using the lvextend command: 

      # lvextend -L+100M /dev/vg001/snap

Additional resources

  • lvcreate(8), lvextend(8), and lvs(8) man pages
  • /etc/lvm/lvm.conf file

10.3. Merging snapshot to its original volume

Use the lvconvert command with the --merge option to merge a snapshot into its original (the origin) volume. You can perform a system rollback if you have lost data or files, or otherwise you have to restore your system to a previous state. After you merge the snapshot volume, the resulting logical volume has the origin volume’s name, minor number, and UUID. While the merge is in progress, reads or writes to the origin appear as they were directed to the snapshot being merged. When the merge finishes, the merged snapshot is removed.

If both the origin and snapshot volume are not open and active, the merge starts immediately. Otherwise, the merge starts after either the origin or snapshot are activated and both are closed. You can merge a snapshot into an origin that cannot be closed, for example a root file system, after the origin volume is activated.

Procedure

  1. Merge the snapshot volume. The following command merges snapshot volume vg001/snap into its origin: 

    # lvconvert --merge vg001/snap
Merging of volume vg001/snap started.
  vg001/origin: Merged: 100.00%

  2. View the origin volume: 

    # lvs -a -o +devices
  LV      VG    Attr       LSize  Pool Origin Data% Meta% Move Log Cpy%Sync Convert Devices
  origin vg001  owi-a-s---  1.00g                                                  /dev/sde1(0)

Additional resources

  • lvconvert(8) man page