11 use of virtual copy snapshots, 1 in synchronous mode, Use of virtual copy snapshots – HP 3PAR Remote Copy Software User Manual

Page 47: In synchronous mode

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Use of Virtual Copy Snapshots

InForm OS Version 2.3.1

Remote Copy User’s Guide

2.11 Use of Virtual Copy Snapshots

Remote Copy uses virtual copy snapshots (point-in-time virtual copies) of a virtual volume to

minimize the amount of data that needs to be sent over the network to resynchronize volumes

that were previously synchronized. There are several scenarios where snapshots are used.

2.11.1 In Synchronous Mode

In synchronous mode, a snapshot is created only under error or recovery situations or when a

group is manually stopped. If the backup storage server fails, or all communication links to the

backup server fail, the primary storage server stops the replication of all volume groups. It also

takes snapshots of all volumes that were completely synchronized. If a volume was still

undergoing the initial full synchronization when the failure happened, a snapshot is not taken

of that volume. That volume fully synchronizes when Remote Copy is restarted.

When the backup storage server comes back up, Remote Copy must be manually restarted

using the

startrcopygroup

command for all volume groups, unless the

auto_recover

policy is in use (see

on page 4.4). When restarted,

Remote Copy first looks for a valid resynchronization snapshot for a volume. If the

resynchronization snapshot exists, Remote Copy resynchronizes the secondary volume by

sending only the differences between that snapshot and the current data in the primary base

volume. But before this resynchronization is started, the system takes snapshots of all the

secondary volumes that were previously synchronized. While the resynchronization is taking

place, the state of the secondary volume becomes

syncing

. During that time, the secondary

volume is not in a consistent state because the updates are written by location order rather

than by time order. When the resynchronization completes, the snapshots on the primary and

the backup servers are deleted.

If the primary server fails during the resynchronization, the secondary base volumes are left in

an inconsistent state, but the snapshots of the secondary volumes are left behind. When the

primary server comes back, the next resynchronization brings the secondary volumes back in

synchronization with the primary volumes.

NOTE: See

information about space allocation and deleted snapshots.