SANRAD V-Switch manual Creating a Snapshot, 144

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 75, 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.

QA Testing

S A N S

N A S H

A M M O

P U N T

Snapshot1

10265

Figure 75. 1st Snapshot Created

Figure 76, page 145 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.

144 SANRAD V-Switch User Manual