HP StorageWorks XP Remote Web Console Software - Manual (Page 33)

When the command executes the process and ends normally, the pair status changes according to the
process. For more information about how the pair status changes when the command ends normally, see
Figure 4 and the corresponding text.
Notes on creating pools
When creating pools, you must be careful about the pool capacity and types of volumes specified as
pool-VOLs. This section describes notes on pool capacity definitions and using external LUs as pool-VOLs.
Defining pool capacity
Pool capacity is the total capacity of pool volumes defined in the pool. If pool capacity is insufficient, the
status of Snapshot XP pairs could change to PSUE. When creating a pool, you must estimate the copy
capacity (for example, the capacity of snapshot data to be copied to the pool) and set enough pool
capacity for the estimated copy capacity. If copy capacity changes according to the time period, set
enough pool capacity for the largest copy capacity.
NOTE:
•
When snapshot data is copied to a pool, a multiple of 256 KB of the pool is used. Even if the
snapshot data capacity is less than 256 KB, you need 256 KB in the pool for the data.
• If the same P-VOL area is updated more than once, snapshot data for the area is copied to the pool
only when the first update occurs. Therefore, when the same P-VOL area is updated only once,
written data capacity is equal to copied data capacity. However, when the same P-VOL area is
updated repeatedly, copy capacity is smaller than written data capacity.
• HP recommends using a volume with a capacity that is a multiple of 256 KB as a pool-VOL. If the
capacity of the volume specified as a pool-VOL is not a multiple of 256 KB, the capacity that is less
than 256 KB is rounded off. For example, if you add a volume with 100.125 MB as a pool-VOL,
125 KB is rounded off and the pool's capacity increases by only 100 MB.
When estimating copy capacity, consider whether there is only one snapshot data or more than one
snapshot data.
• For one snapshot data
If the same P-VOL area is updated only once, copy capacity is equal to the capacity of data written
between storing and deleting snapshot data.
For example, to make a backup using Snapshot XP, you can back up S-VOL data after storing the
snapshot data, and then delete the snapshot data. In this case, only data written during the backup
must be copied to the pool, and there is only one snapshot data. Therefore, you can make a backup
with a pool with small capacity. However, since you must read a large amount of S-VOL data during
the backup, access to the P-VOL increases and, consequently, host I/O performance might be reduced.
• For multiple snapshot data
After storing each snapshot data, if the same P-VOL area is updated only once until the snapshot
data is deleted, copy capacity is equal to the capacity of data written between storing and deleting
the snapshot data. If there is more than one P-VOL, copy capacity is equal to the total capacity of
data written to each P-VOL.
NOTE:
When storing more than one snapshot data in the pool, some Snapshot XP pairs might share snapshot
data in the pool. While snapshot data is shared, you can delete pairs, but cannot delete shared snapshot
data from the pool. Shared snapshot data is not deleted from the pool until all Snapshot XP pairs
sharing the snapshot data are deleted.
HP StorageWorks Snapshot XP user guide 33