HP StorageWorks XP Remote Web Console Software - Manual (Page 20)

20 ShadowImage for the XP128/XP1024/XP10000/XP12000
NOTE: Update copy operations do not occur every time a host issues a write I/O operation to the S-VOL
of a zSeries and SI390 pair. Update copy operations are performed asynchronously according to the
differential bitmap, which is stored in shared memory. If shared memory is lost (for example, offline micro
exchange or volatile PS on), the differential bitmap is also lost. In this case, the
XP128/XP1024/XP10000/XP12000 treats the entire S-VOL (T-VOL for resync-R pairs) as difference data
and recopies all data to the T-VOL (S-VOL for resync-R pairs) to ensure proper pair resynchronization. For
pairs with SP-pend or V-Split status, the XP128/XP1024/XP10000/XP12000 changes the status to
suspend due to the loss of the differential bitmap, ensuring proper resynchronization of these pairs. If
shared memory has been lost, allow extra time for SI390 operations.
Figure 3 Update Copy operation
Split Pair Operation
The SI390 split capability provides point-in-time backup of your data and also facilitates real data testing
by making the SI390 copies (T-VOLs) available for host access. The SI390 split operation copies whole
S-VOL data or differential data at that time to the T-VOL to make the T-VOL identical to the state of the
S-VOL when the split command was issued. When the split operation is complete, the pair status changes
to split. Note that if there are many host I/Os for the S-VOL, it may take time to synchronize the S-VOL and
the T-VOL.
While the pair is split, the disk array establishes a differential bitmap for the split S-VOL and T-VOL, and
records all updates to both volumes. When a split pair is resynced, the disk array copies all flagged tracks
from the S-VOL to the T-VOL. This method ensures that the S-VOL and T-VOL are correctly resynchronized
and also reduces the time needed to resynchronize the pair. The S-VOL remains fully accessible during the
split operation. Whether you can access the T-VOL depends on what split type is specified. For details on
the split type, see ” Split Type ” on page 21. Split operations cannot be performed on suspend pairs.
You can split existing SIz pairs as needed, and you can also use the split pair operation to create and split
pairs in one step. When you split an existing pair, only the differential data of the S-VOL at that time will
be copied to the T-VOL. When you create a new pair and then split that pair, whole S-VOL data will be
copied to the T-VOL. Use the Split Volume Pair window (see ” Splitting ShadowImage Pairs ” on page 54) to
split existing SI390 pairs, or to add and split new SI390 pairs in one step.
Copy Pace
When splitting pairs, you can select the pace for the pending update copy operation(s): slower, medium,
and faster. The slower pace minimizes the impact of SI390 operations on disk array I/O performance
while the faster pace splits the pairs as quickly as possible.