HP XP Racks - Manual (Page 20)

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Before creating multiple pairs during the Create Pairs operation, make sure to set up S-VOL
LDEVs to allow the system to correctly match them to P-VOLs.
You must do this because, even though you select multiple volumes as P-VOLs, you specify
just one S-VOL. The system automatically assigns subsequent secondary system LDEVs as
S-VOLs based on the option you specify for Selection Type. These are the options:
◦ Interval: The interval you specify will be skipped between LDEV numbers in the secondary
system.
For example, suppose you specify LU 01 as the initial (base) S-VOL, and specify 3 for
Interval. This results in secondary system LDEV 04 being assigned to the next P-VOL, 07
assigned to the subsequent P-VOL, and so on. To use Interval, you set up secondary system
LDEV numbers according to the interval between them.
◦ Relative Primary Volume. The difference is calculated between the LDEV numbers of two
successive P-VOLs. S-VOLs are assigned according to the closest LUN number.
For example, if the LUN numbers of three P-VOLs are 1, 5, and 6; and you set LUN
numbers for the initial S-VOL (Base Secondary Volume) at 2, the LUN numbers of the
three S-VOLs will be set at 2, 6, and 7, respectively.
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Because the contents of the P-VOL and S-VOL are identical, the S-VOL can be considered a
duplicate of the P-VOL. Because the host operating system does not allow duplicate volumes,
the host system administrator must take precautions to prevent system problems related to
duplicate volumes. You must define the 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 paircreate operation will
fail.
CAUTION: When S-VOLs and P-VOLs are connected to the same hosts, it is strongly
recommended that you define the S-VOLs to remain offline. This is because under this condition,
the S-VOL is usually offline when a pair is released. 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 error.
About allowing 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 re-synchronize the pair.
Enabling S-VOL-write is done during the pairsplit operation.
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The S-VOL write option is available when the split operation is performed from the primary
system.
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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 ensuring proper
resynchronization.
Allowing I/O to the P-VOL after a split—Fence Level options
You can specify whether the host is denied access or continues to access the P-VOL when the pair
is split due to an error. This is done with the Primary Volume Fence Level setting. You specify one
20 Planning for Continuous Access Synchronous