Defining pool capacity – HP StorageWorks XP Remote Web Console Software User Manual
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Defining pool capacity
Pool capacity is the total capacity of pool volumes defined in the pool. If pool capacity is insufficient,
the status of XP Snapshot 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 XP Snapshot, 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 XP Snapshot 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 XP Snapshot
pairs sharing the snapshot data are deleted.
Although you estimate copied data capacity and set pool capacity according to the information in
this section, if pool capacity exceeds the threshold, implement the measures described in
“
Using XP Snapshot
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