HP XP P9500 Storage - Manual (Page 21)

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.
•
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.
Fence Level options for I/O to the P-VOL after suspension
The P-VOL Fence Level setting determines whether the host is denied access or continues to access
the P-VOL when the pair is suspended due to an error. You specify one of the following Fence
Level options during the initial copy and resync operations. You can also change the Fence Level
option outside these operations.
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Data – The P-VOL is fenced if an update copy operation fails. This prevents the host from
writing to the P-VOL during a failure. This setting should be considered for the most critical
volumes for disaster recovery. This setting reduces the amount of time required to analyze the
currency of the S-VOL during disaster recovery efforts. This setting is also designed for
applications that can continue to operate with another device pair.
•
Status – The P-VOL is fenced only if the primary system is not able to change S-VOL status to
suspended when an update copy operation fails. If the primary system successfully changes
S-VOL pair status to suspended, subsequent write I/O operations to the P-VOL will be accepted,
and the system will keep track of updates to the P-VOL. This allows the pair to be resynchronized
quickly. This setting also reduces the amount of time required to analyze S-VOL currency during
disaster recovery.
•
Never – The P-VOL is never fenced. This setting should be used when I/O performance
out-weighs data recovery. “Never” ensures that the P-VOL remains available to applications
for updates, even if all Continuous Access Synchronous copy operations have failed. The
S-VOL may no longer be in sync with the P-VOL, but the primary system keeps track of updates
to the P-VOL while the pair is suspended. Host failover capability is essential if this fence level
setting is used. For disaster recovery, the currency of the S-VOL is determined by using the
sense information transferred via host failover or by comparing the S-VOL contents with other
files confirmed to be current.
Differential data options
You 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. It is then restored to the S-VOL.
Planning for pairs and pair volumes 21