Performance considerations, Notes on defining pool capacity – HP StorageWorks XP Remote Web Console Software User Manual
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According to the result above, the maximum number of XP Snapshot pairs that can be created is
1,018,575.
Performance Considerations
If XP Snapshot performs restore operation when the load of the subsystem is heavy, it is more likely that
host server I/O performance (response) may be degraded. The Copy Threshold option allows you to
effectively decrease the load of the subsystem. For details, see “
Notes on Defining Pool Capacity
The pool capacity is the total capacity of the pool volumes that are defined in the pool. If the pool
capacity is not enough, the status of the XP Snapshot pairs could change to PSUE. When you create a
pool, you must estimate the copy capacity (that is, the capacity of the snapshot data to be copied to the
pool) and set enough pool capacity for the estimated copy capacity. If the copy capacity would change
according to the period of time, set enough pool capacity for the largest copy capacity.
Notes
:
•
When snapshot data is copied to a pool, a multiple of 256 kB of the pool will be consumed.
Even if the capacity of the snapshot data is less than 256 kB, you need a 256 kB area in the
pool for the data.
•
If the same area in the P-VOL is updated more than one time, the snapshot data for the area
is copied to the pool only when the first update has taken place. Therefore, when the same
area in the P-VOL is updated only once, the written data capacity is equal to the copied data
capacity. However, when the same area in the P-VOL is updated repeatedly, the copy capacity
will be smaller than the written data capacity.
•
HP recommends that you use a volume whose capacity is a multiple of 256 kB as a pool-VOL. If
the capacity of the volume that is specified as a pool-VOL is not a multiple of 256 kB, the capacity
that is less than 256 kB will be rounded off. For example, if you add a volume of 100.125 MB as a
pool-VOL, 125 kB will be rounded off and the capacity of the pool will increase only by 100 MB.
When you estimate the copy capacity, you need to consider whether there is only one snapshot data or
more than one snapshot data.
•
In case of one snapshot data
If the same area in the P-VOL is updated only once, the copy capacity is equal to the capacity of
the data that is written between the storing and the deletion of the snapshot data.
For example, if you want make a backup by using XP Snapshot, you can back up the data from
the S-VOL after storing the snapshot data, and then delete the snapshot data. In this case, only
the data that was written during the backup needs to be copied to the pool, and there is only
one snapshot data. Therefore, you can make a backup with the pool that has a small capacity.
However, since you need to read a large amount of data from the S-VOL during the backup, the
access to the P-VOL increases and consequentially the host I/O performance may be lowered.
•
In case of multiple snapshot data
After storing each snapshot data, if the same area in the P-VOL is updated only once until the
snapshot data is deleted, the copy capacity is equal to the capacity of the data that is written
between the storing and the deletion of the snapshot data. If there is more than one P-VOL, the
copy capacity is equal to the total capacity of the data written to each P-VOL.
Note:
When you store more than one snapshot data in the pool, some XP Snapshot pairs may
share snapshot data in the pool. While the snapshot data is shared, you can delete the pairs
but cannot delete the shared snapshot data from the pool. The shared snapshot data will not be
deleted from the pool until all the XP Snapshot pairs that share the snapshot data are deleted.
Though you estimate the copied data capacity and set the pool capacity according to the notes in this
section, if the pool capacity exceeds the threshold, see “
” on page 73 and
implement measures.
XP24000 Snapshot User's Guide
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