Understanding Volume Snapshots - Novell LINUX ENTERPRISE SERVER 11 - STORAGE ADMINISTRATION GUIDE 2-23-2010 Administration Manual

Volume Snapshots
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A file system snapshot is a copy-on-write technology that monitors changes to an existing volume's
data blocks so that when a write is made to one of the blocks, the block's value at the snapshot time
is copied to a snapshot volume. In this way, a point-in-time copy of the data is preserved until the
snapshot volume is deleted.
Section 14.1, "Understanding Volume Snapshots," on page 131
Section 14.2, "Creating Linux Snapshots with LVM," on page 132
Section 14.3, "Monitoring a Snapshot," on page 132
Section 14.4, "Deleting Linux Snapshots," on page 132

14.1 Understanding Volume Snapshots

A file system snapshot contains metadata about and data blocks from an original volume that have
changed since the snapshot was taken. When you access data via the snapshot, you see a point-in-
time copy the original volume. There is no need to restore data from backup media or to overwrite
the changed data.
In a Xen host environment, the virtual machine must be using an LVM logical volume as its storage
back-end, as opposed to using a virtual disk file.
Linux 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 backup 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 original 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 original block. As blocks change from their original value, the snapshot size grows.
When you are sizing the snapshot, consider how much data is expected to change on the original
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 original 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 original volume will change at least one time before you delete the snapshot, then the
snapshot volume should be at least as large as the original 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.
IMPORTANT: During the snapshot's lifetime, the snapshot must be mounted before its original
volume can be mounted.
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 original volume. When the snapshot is
full, it is disabled, which prevents you from remounting the original volume.
Remove snapshots in a last created, first deleted order.
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