Planning I/O To The S-Vol; Managing Differential Data - HP StorageWorks P9000 User Manual

When creating multiple pairs concurrently, make sure that you set up S-VOL LUNs in a way
that allows the system to correctly match them to your selected P-VOLs.
Even though you select multiple volumes as P-VOLs in the Remote Web Console Pair create
procedure, only one of them displays in the dialog box—the one with the lowest LUN. Because
of this, you are able to specify only one S-VOL. The system automatically assigns LUs on the
secondary system as S-VOLs for the other selected P-VOLs according to LUN.
For example, three LUs have been set up on the secondary system to become an S-VOL, named
LUN001, LUN002, and LUN003. During the Paircreate operation, you select three volumes
as P-VOLs, and then select LUN001 as the S-VOL. The system will then assign LUN002 and
LUN003 as the S-VOL for the other two P-VOLs. Therefore, make sure that you assign LUNs
to your S-VOL that will be selected in the proper order.
Since the contents of the P-VOL and S-VOL of a Continuous Access Synchronous pair are
identical, the S-VOL can be considered a duplicate of the P-VOL. Since the host operating
system (OS) does not allow duplicate volumes, the host system administrator must take
precautions to prevent system problems related to duplicate volumes. You must define the
Continuous Access Synchronous S-VOLs so they do not auto mount or come online to the same
host at the same time as the P-VOLs.
Continuous Access Synchronous does not allow the S-VOL to be online (except when the pair
is split). If the S-VOL is online, the Continuous Access Synchronous pair create operation will
fail.
CAUTION:
host server(s) as the Continuous Access Synchronous P-VOLs, the following problem can occur:
◦ When a Continuous Access Synchronous pair is released, the old S-VOL is usually offline.
If the host is then restarted, the system administrator may be offered both volumes and
asked which volume should be left offline. This can be confusing and is prone to result
in an error.
If the Continuous Access Synchronous S-VOLs and P-VOLs are connected to the same host(s),
HP strongly recommends that the S-VOLs are defined to remain offline to avoid this problem.

Planning I/O to the S-VOL

The secondary system rejects write I/O to the S-VOL, unless the S-VOL-write option is enabled.
Then, read and write I/O is allowed to the S-VOL while the pair is split. In this instance, S-VOL
and P-VOL track maps keep track of differential data and are used to resynchronize the pair.
Enabling S-VOL-write is done during the pairsplit operation.
The S-VOL write option is available when the split operation is performed from the primary
system.
When you resync a pair with the S-VOL write option enabled, the secondary system sends
S-VOL differential data to the primary system. This data is merged with P-VOL differential data,
and out-of-sync tracks are determined and updated on both systems, thus insuring proper
resynchronization.

Managing differential data

You will have the opportunity to specify how differential data is managed during the Paircreate
operation. Differential data is update data that is collected when the pair is split. This data is stored
in units of tracks or cylinders until the pair is resynchronized. Then it is restored to the S-VOL.
18 Planning for Continuous Access Synchronous
If the Continuous Access Synchronous S-VOLs are physically attached to the same