Red Hat GLOBAL FILE SYSTEM 5.0, Руководство администратора

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as it was prior to the change so that it can reconstruct the state of the device.
Note
LVM snapshots are not supported across the nodes in a cluster.
Because 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.
Note
Snapshot copies of a file system are virtual copies, not actual media backup for a
file system. Snapshots do not provide a substitute for a backup procedure.
If a snapshot runs full, the snapshot is dropped. This is to be sure that there is enough space for
the origin file system. You should regularly monitor the size of the snapshot. Snapshots are fully
resizeable, however, so if you have the storage capacity you can increase the size of the
snapshot volume to prevent it from getting dropped. Conversely, if you find that 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.
When you create a snapshot file system, full read and write access to the origin stays possible.
If a chunk on a snapshot is changed, that chunk is marked and never gets copied from the
original volume.
There are several uses for the snapshot feature:
• Most typically, a snapshot is taken 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 whether the original file system requires file system repair.
• Because the snapshot is read/write, you can test applications against production data by
taking a snapshot and running tests against the snapshot, leaving the real data untouched.
• You can create volumes for use with the Xen virtual machine monitor. You can use the
snapshot feature to create a disk image, snapshot it, and modify the snapshot for a particular
domU instance. You can then create another snapshot and modify it for another domU
instance. Since the only storage used is chunks that were changed on the origin or snapshot,
the majority of the volume is shared.
Snapshot Volumes
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