SANRAD I3.1.1205 - Manual (Page 160)

8-8 SANRAD V-Switch CLI User Manual
Creating a Snapshot
You can create a snapshot, a point-in-time copy, of any volume at the top of a
hierarchy using the CLI command volume create snapshot. A snapshot
does not create a full copy of its source volume. It is a dynamic and dependent
volume that stores the original data from a source volume when changes to the
source volume are made after the snapshot’s creation. Snapshot volumes use
the copy old on write method, copying the old source data to the snapshot and
only then writing new data to the source volume.
Snapshots can be used for serverless backup, reducing the load on the
application server. The backup copy from a snapshot is a full copy of the
source volume at the time of the snapshot and adequate size must be allocated
for the backup volume. A snapshot can be built on any volume hierarchy but,
once created, cannot be used as a child in another volume hierarchy. This
means that a snapshot can be created on a mirror volume with two children.
However, after the snapshot is created, a mirror volume cannot be created,
using the snapshot as one of the children, to copy the snapshot volume.
Figure 8-4, shows a source volume with its snapshot when the snapshot is first
created. Initially, a snapshot is empty because there has not yet been a change
in its source volume. Only when a write operation is performed on the source
volume will the snapshot begin to fill up.
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Snapshot1 QA Testing
M M A O
A S N H
A N S S
U N P T
Figure 8-4. 1
st
Snapshot Created
Figure 8-4 shows the same source and snapshot volume after a write operation
to sector 1. The snapshot records the original data from sector 1, S, to the
snapshot and then the new data, B, is written to the source volume.