HP 3PAR Operating System Software - Manual (Page 270)

29 Remote-Copy Snapshots
See the following: For information about:
“Why Use Snapshots” (page 270) What snapshots are and why remote copy uses them
“Volume Space Considerations” (page 270) Ensuring virtual volumes have enough space to create the
necessary snapshots
“Snapshots and Initial Synchronization Failure” (page 271) In synchronous mode, when remote copy takes snapshots
and what it does with them
“Snapshots and Resynchronization” (page 271)
“Snapshots and Resynchronization Failure in Synchronous
Mode” (page 271)
“Snapshots Taken before or during Disaster Recovery”
(page 272)
“Snapshots Taken during Initial Synchronization” (page 272) In asynchronous periodic mode, when remote copy takes
snapshots and what it does with them
“Snapshots Taken during Resynchronization” (page 272)
“Snapshots and Resynchronization Failure in Asynchronous
Periodic Mode” (page 273)
“Snapshots Taken before or during Disaster Recovery”
(page 273)
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 created mainly 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 backup systems.
Remote copy automatically creates snapshots when a system, or all links, go down as a result of
failure or scheduled downtime.
Volume Space Considerations
You should use remote copy only with virtual volumes that can automatically draw space from a
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snapshot becomes outdated (stale).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, with the
primary and secondary volumes back in synchronization, you might need to perform a full
resynchronization.
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”
(page 122) .
270 Remote-Copy Snapshots