HP XP P9500 Storage - Manual (Page 15)

pair or journal returns to Paired status when the primary and secondary storage systems are
re-synchronized.
Read and Write I/O operations during remote copy operation
The primary system reads from the P-VOL when it receives a read I/O. If the read fails, the
redundancy provided by RAID-1 or RAID-5 technology recovers the failure. The primary system
does not read the S-VOL for recovery.
When a primary system receives a write I/O for a P-VOL in PAIR status, the system performs the
update copy operation, as well as writing to the P-VOL. The write operation completes independently
of the update copy operations on the S-VOL.
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. If the update copy operation fails, the secondary system suspends
either the affected pair or all Continuous Access Journal pairs in the journal, depending on the
type of failure. When the suspended pair or journal is resumed, the primary and secondary systems
negotiate the resynchronization of the pair(s).
During normal Continuous Access Journal operations, the secondary system does not allow S-VOLs
to be online (mounted). Therefore, hosts cannot read from or write to S-VOLs. The S-VOL write
enable option allows write access to a secondary data volume while the pair is split. The option
is only available when you split the pair from the primary system.
To reduce the overhead associated with these remote copy activities and maximize data transfer,
the P9500 storage system utilizes a special write command, which is allowed only for Continuous
Access Journal initial and update copy operations. This command transfers the control parameters
and the FBA format data for consecutive updated records in a track using a single write operation.
The special Continuous Access Journal write command eliminates the overhead required for
performing FBA to CKD and CKD to FBA conversions.
Remember that the host cannot write data to the Continuous Access Journal P-VOL belonging to
the journal that was registered when 2DC Cascade is set to Enable in the Edit Journal Volume dialog
box.
S-VOL write option
When a pair is split, you can set an option that will allow write I/O to the S-VOL. The S-VOL write
option is selected during the Suspend Pair operation When performing the operation from the
primary system. 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.
Difference management
Differential data (updates during split or suspension) between the P-VOL and S-VOL is stored in a
track bitmap. When a split/suspended 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 storage system.
Pair status
Every pair operation results in a change in pair status. You should monitor pair status to insure
that an operation completed successfully. Also, pairs must have a specific status in order for specific
operations to be executed.
Read and Write I/O operations during remote copy operation 15