Read And Write I/O Operations During Remote Copy Operation; S-Vol Write Option; S-Vol Read Option; Difference Management - HP XP P9500 User Manual

If an update copy operation fails, the remote system suspends the affected pair or all Continuous
Access Synchronous Z pairs in the journal. This is dependent on the type of failure. The
suspended pair or journal returns to Paired status when the primary and secondary storage
systems are resynchronized.

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 RAID1 or RAID5 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 Duplex 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 Z 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 Z 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.
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.

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.

S-VOL read option

You can also have a read option enabled for the S-VOL (mode 20) while the pair is suspended.
The secondary system allows you to change the VOLSER of suspended S-VOLs. By setting the mode
20 and the mode 190 to ON, you can change the VOLSER and VTOC of suspended S-VOLs. This
allows the S-VOL to be online to the same host as the P-VOL during the suspend. The primary system
copies the P-VOL VOLSER and VTOC back to the S-VOL when the pair is resynchronized. Make
sure any secondary volumes that were varied online to the host are varied offline before performing
the resync operating. Write I/Os are rejected by the secondary storage system during the suspend.
The primary system copies the P-VOL VOLSER back 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 a S-VOL.

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.
Read and Write I/O operations during remote copy operation 15