HP XP7 Storage - Manual (Page 15)

The secondary system updates the S-VOL according to the write sequence number in the journal
data. This maintains data consistency between P-VOL and S-VOL.
If the P-VOL write operation fails, the primary system reports a unit check and does not create the
journal data for this operation. As mentioned, if the update copy operation fails, the secondary
system suspends either the affected pair or all Continuous Access Journal Z pairs in the journal,
depending on the type of failure. When the suspended pair or journal is resynchronized, the
primary and secondary systems negotiate the resynchronization of the pairs.
During normal operations, the secondary system does not allow S-VOLs to be online (mounted).
Therefore, hosts cannot read from and write to S-VOLs. However, if the S-VOL write option is
enabled, write access to an S-VOL is allowed while the pair is split. The pair must be split from the
primary system for the option to take effect.
When you issue the DEVSERV command to the Continuous Access Journal Z S-VOL, INDETERMINATE
FAILING UNIT is returned if the status of Continuous Access Journal Z S-VOL is online.
INTERVENTION REQUIRED is returned, if the status of Continuous Access Journal Z S-VOL is offline.
Differential data management
Differential data is the data that is changed in the P-VOL when a pair is suspended and that is not
reflected in the S-VOL. This data is stored in a track bitmap. When the pair is resumed, the primary
system merges the P-VOL and S-VOL bitmaps, and the differential data is copied to the S-VOL.
The number of bitmap areas affects the maximum possible number of pairs that can be created in
the system.
S-VOL write option
When splitting a pair, you can set an option allowing write I/O to the S-VOL. When you
resynchronize a split pair whose S-VOL is write-enabled, the secondary system sends the S-VOL
track bitmap to the primary system, which merges the P-VOL and S-VOL bitmaps to determine which
tracks are out of sync. This ensures proper resynchronization of the pair.
S-VOL read option
You can change the VOLSER of suspended S-VOLs when system option mode 20 is set to ON.
This read options allows the S-VOL to be online to the P-VOL’s host. The read option is enabled
while the pair is suspended.
Write updates are rejected by the secondary system during this time either by the command being
rejected or by required intervention.
The primary system copies the P-VOL VOLSER and VTOC to the S-VOL when the pair is resumed.
When the S-VOL read option is not enabled and/or the pair is not suspended, the secondary
system rejects read and write I/O to the S-VOL by the command being rejected or by required
intervention.
Pair status
Every pair operation results in a change in pair status. Also, when you want to perform an operation,
the pair must have a specific status in order to for the operation to run.
You will monitor pair status to ensure that you can perform the desired operation, and to ensure
that an operation completed successfully.
To become familiar with the operations that result in the pair statuses, it is recommended that you
review
“Pair status definitions” (page 80) .
Reference Information 15