Hitachi ShadowImage for z/OS user guide (HIT9503-96007, October 2007)

ShadowImage for z/OS user guide 25
CAUTION: Even if the copy operation is completed without any host I/O, the data in the S-VOL and the
data in the T-VOL may not be the same. Whether the S-VOL and the T-VOL have the same data depends on
the condition of the disk array. To make the S-VOL data and the T-VOL data equal, split the pair and make
the pair status split.
Backward Pairresync Operation
Backward pairresync operation resynchronizes pairs by copying data on T-VOLs to S-VOLs. The following
are the types of the backward pairresync operations:
• Reverse. The reverse resync operation (see Figure 5) synchronizes the S-VOL with the T-VOL. The copy
direction for a reverse resync operation is T-VOL to S-VOL. Table 5 on page 26 lists the operational
requirements for the reverse resync operation. The pair status during a reverse resync operation is
resync-r, and the S-VOL and T-VOL become inaccessible to all hosts for write operations during a
reverse resync operation. As soon as the reverse resync operation is complete, the S-VOL becomes
accessible. The reverse resync operation can be performed only on split pairs, not on suspended pairs.
• Quick Restore. The quick restore operation (see Figure 5) speeds up the reverse resync operation by
changing the volume map in the XP1024/XP128/XP12000/XP10000/SVS 200 to swap the contents
of the S-VOL and T-VOL without copying the T-VOL data to the S-VOL. The S-VOL and T-VOL are
resynchronized when update copy operations are performed for pairs in the duplex status. The pair
status during a quick restore operation is resync-r until the volume map change is complete. The S-VOL
and T-VOL become inaccessible to all hosts for write operations during a quick restore operation.
Table 5 on page 26 lists the operational requirements for the quick restore operation.
Figure 5 Backward pairresync operations
During the quick restore operation, the RAID levels, Cache LUN XP settings, and HDD types of the S-VOL
and T-VOL are exchanged. For example, if the S-VOL has a RAID-1 level and the T-VOL has a RAID-5 level,
the quick restore operation changes the RAID level of the S-VOL to RAID-5 and of the T-VOL to RAID-1. This
also applies to RAID-6 volumes. To avoid any performance impact due to the quick restore operation:
1. Verify that the S-VOL and T-VOL have the same RAID level and HDD type before performing the quick
restore operation. If you want to restore the original RAID levels after quick restore, stop host I/Os to the
pair, split the pair, perform the quick restore operation for that pair again, and then restart the host
I/Os to the pair.
2. Because the Cache LUN XP settings are exchanged during a quick restore operation, you must perform
one of the two following operations. If you do not, the change of location of the cache residence areas
may cause I/O performance to the Cache LUN XP data to be down.
a. Set the same Cache LUN XP settings (locations) for the S-VOL and T-VOL before performing the
quick restore operation.
b. Release the Cache LUN XP settings of the S-VOLs and T-VOLs before the quick restore operation,
and then reset the Cache LUN XP settings of the source and target volumes after the pair changes to
duplex status as a result of the quick restore operation.