Operating System Software Owner manual

30 Remote Copy Snapshots
See:For information on:
“Why Use Snapshots” (page 203)What snapshots are and why Remote Copy uses them
“Volume Space Considerations” (page 203)Ensuring virtual volumes have enough space to create the
necessary snapshots
“Snapshots and Initial Synchronization Failure” (page 204)In synchronous mode, when Remote Copy takes snapshots
and what it does with them
“Snapshots and Resynchronization” (page 204)
“Snapshots and Resynchronization Failure” (page 204)
“Snapshots Taken before or during Disaster Recovery”
(page 204)
“Snapshots Taken during Initial Synchronization” (page 205)In asynchronous periodic mode, when Remote Copy takes
snapshots and what it does with them
“Snapshots Taken during Resynchronization” (page 205)
“Snapshots and Resynchronization Failure” (page 206)
“Snapshots Taken before or during Disaster Recovery”
(page 206)
Overview of Snapshots
Why Use Snapshots
A snapshot is a point-in-time virtual copy of a virtual volume. Under normal operating conditions,
a snapshot includes only new data saved to the primary virtual volume since the last synchronization
with the secondary virtual volume.
• Snapshots are mainly created to keep track of the updates to the primary volume when new
data is not immediately sent to the secondary volume; for example, in asynchronous periodic
volume groups.
• To minimize the amount of data sent over the network to secondary volumes during
resynchronization, Remote Copy creates snapshots of the updates to the primary volumes and
sends only those snapshots to the backup system(s).
• Remote Copy also automatically creates snapshots when a system or all links go down as a
result of failure or scheduled downtime.
Volume Space Considerations
You should only use Remote Copy with virtual volumes that can automatically draw space from a
Common Provisioning Group (CPG). If the Remote Copy virtual volumes do not automatically draw
space from a CPG, the volumes can run out of data and snapshot space.
If the snapshot space on a virtual volume is insufficient (and the volume is not configured to
automatically update) and Remote Copy becomes unable to create new snapshots, the last existing
snapshot becomes outdated (stale). To return your system to a consistent, up-to-date state, you
might need to perform a full resynchronization to bring the primary and secondary volumes back
in sync.
The amount of snapshot space required per virtual volume depends on the kind of synchronization
performed (full synchronization or resynchronization). For more information, see “Synchronization
Types and Limits” (page 197).
Additional Information:
• “Common Provisioning Groups” (page 187)
• “Converting Standard Virtual Volumes” (page 95)
Overview of Snapshots 203