6 LVM volume snapshots #

A file system snapshot contains metadata about itself and data blocks from a source logical volume that has changed since the snapshot was taken. When you access data via the snapshot, you see a point-in-time copy of the source logical volume. There is no need to restore data from backup media or to overwrite the changed data.

Important: Mounting volumes with snapshots

During the snapshot’s lifetime, the snapshot must be mounted before its source logical volume can be mounted.

LVM volume snapshots allow you to create a backup from a point-in-time view of the file system. The snapshot is created instantly and persists until you delete it. You can back up the file system from the snapshot while the volume itself continues to be available for users. The snapshot initially contains some metadata about the snapshot, but no actual data from the source logical volume. Snapshot uses copy-on-write technology to detect when data changes in an original data block. It copies the value it held when the snapshot was taken to a block in the snapshot volume, then allows the new data to be stored in the source block. As more blocks change from their original value on the source logical volume, the snapshot size grows.

When you are sizing the snapshot, consider how much data is expected to change on the source logical volume and how long you plan to keep the snapshot. The amount of space that you allocate for a snapshot volume can vary, depending on the size of the source logical volume, how long you plan to keep the snapshot, and the number of data blocks that are expected to change during the snapshot’s lifetime. The snapshot volume cannot be resized after it is created. As a guide, create a snapshot volume that is about 10% of the size of the original logical volume. If you anticipate that every block in the source logical volume will change at least one time before you delete the snapshot, then the snapshot volume should be at least as large as the source logical volume plus some additional space for metadata about the snapshot volume. Less space is required if the data changes infrequently or if the expected lifetime is sufficiently brief.

In LVM2, snapshots are read/write by default. When you write data directly to the snapshot, that block is marked in the exception table as used, and never gets copied from the source logical volume. You can mount the snapshot volume, and test application changes by writing data directly to the snapshot volume. You can easily discard the changes by dismounting the snapshot, removing the snapshot, and then remounting the source logical volume.

In a virtual guest environment, you can use the snapshot function for LVM logical volumes you create on the server’s disks, as you would on a physical server.

In a virtual host environment, you can use the snapshot function to back up the virtual machine’s storage back-end, or to test changes to a virtual machine image, such as for patches or upgrades, without modifying the source logical volume. The virtual machine must be using an LVM logical volume as its storage back-end, as opposed to using a virtual disk file. You can mount the LVM logical volume and use it to store the virtual machine image as a file-backed disk, or you can assign the LVM logical volume as a physical disk to write to it as a block device.

Beginning in SLES 11 SP3, an LVM logical volume snapshot can be thinly provisioned. Thin provisioning is assumed if you create a snapshot without a specified size. The snapshot is created as a thin volume that uses space as needed from a thin pool. A thin snapshot volume has the same characteristics as any other thin volume. You can independently activate the volume, extend the volume, rename the volume, remove the volume, and even snapshot the volume.

Important: Thinly provisioned volumes in a cluster

To use thinly provisioned snapshots in a cluster, the source logical volume and its snapshots must be managed in a single cluster resource. This allows the volume and its snapshots to always be mounted exclusively on the same node.

When you are done with the snapshot, it is important to remove it from the system. A snapshot eventually fills up completely as data blocks change on the source logical volume. When the snapshot is full, it is disabled, which prevents you from remounting the source logical volume.

If you create multiple snapshots for a source logical volume, remove the snapshots in a last created, first deleted order.

The Logical Volume Manager (LVM) can be used for creating snapshots of your file system.

Open a terminal and enter

> sudo lvcreate -s [-L <size>] -n SNAP_VOLUME SOURCE_VOLUME_PATH

If no size is specified, the snapshot is created as a thin snapshot.

For example:

> sudo lvcreate -s -L 1G -n linux01-snap /dev/lvm/linux01

The snapshot is created as the /dev/lvm/linux01-snap volume.

Open a terminal and enter

> sudo lvdisplay SNAP_VOLUME

For example:

> sudo lvdisplay /dev/vg01/linux01-snap

--- Logical volume ---
  LV Name                /dev/lvm/linux01
  VG Name                vg01
  LV UUID                QHVJYh-PR3s-A4SG-s4Aa-MyWN-Ra7a-HL47KL
  LV Write Access        read/write
  LV snapshot status     active destination for /dev/lvm/linux01
  LV Status              available
  # open                 0
  LV Size                80.00 GB
  Current LE             1024
  COW-table size         8.00 GB
  COW-table LE           512
  Allocated to snapshot  30%
  Snapshot chunk size    8.00 KB
  Segments               1
  Allocation             inherit
  Read ahead sectors     0
  Block device           254:5

Open a terminal and enter

> sudo lvremove SNAP_VOLUME_PATH

For example:

> sudo lvremove /dev/lvmvg/linux01-snap

Using an LVM logical volume for a virtual machine’s back-end storage allows flexibility in administering the underlying device, such as making it easier to move storage objects, create snapshots, and back up data. You can mount the LVM logical volume and use it to store the virtual machine image as a file-backed disk, or you can assign the LVM logical volume as a physical disk to write to it as a block device. You can create a virtual disk image on the LVM logical volume, then snapshot the LVM.

You can leverage the read/write capability of the snapshot to create different instances of a virtual machine, where the changes are made to the snapshot for a particular virtual machine instance. You can create a virtual disk image on an LVM logical volume, snapshot the source logical volume, and modify the snapshot for a particular virtual machine instance. You can create another snapshot of the source logical volume, and modify it for a different virtual machine instance. The majority of the data for the different virtual machine instances resides with the image on the source logical volume.

You can also leverage the read/write capability of the snapshot to preserve the virtual disk image while testing patches or upgrades in the guest environment. You create a snapshot of the LVM volume that contains the image, and then run the virtual machine on the snapshot location. The source logical volume is unchanged, and all changes for that machine are written to the snapshot. To return to the source logical volume of the virtual machine image, you power off the virtual machine, then remove the snapshot from the source logical volume. To start over, you re-create the snapshot, mount the snapshot, and restart the virtual machine on the snapshot image.

The following procedure uses a file-backed virtual disk image and the Xen hypervisor. You can adapt the procedure in this section for other hypervisors that run on the SUSE Linux Enterprise platform, such as KVM. To run a file-backed virtual machine image from the snapshot volume:

Snapshots can be useful if you need to roll back or restore data on a volume to a previous state. For example, you might need to revert data changes that resulted from an administrator error or a failed or undesirable package installation or upgrade.

You can use the lvconvert --merge command to revert the changes made to an LVM logical volume. The merging begins as follows:

After a merge begins, the merge continues automatically after server restarts until it is complete. A new snapshot cannot be created for the source logical volume while a merge is in progress.

While the merge is in progress, reads or writes to the source logical volume are transparently redirected to the snapshot that is being merged. This allows users to immediately view and access the data as it was when the snapshot was created. They do not need to wait for the merge to complete.

When the merge is complete, the source logical volume contains the same data as it did when the snapshot was taken, plus any data changes made after the merge began. The resulting logical volume has the source logical volume’s name, minor number, and UUID. The source logical volume is automatically remounted, and the snapshot volume is removed.