Performance considerations, Notes on defining pool capacity – HP StorageWorks XP Remote Web Console Software User Manual
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Performance Considerations
If XP Snapshot performs a 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.
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.
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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.
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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.
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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 how many generations of snapshot
data you would like to take.
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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, because you need to read a large amount of data from the S-VOL during
the backup, the access to the P-VOL increases and consequently the host I/O performance
may be lowered.
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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.
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
and
implement measures.
40
Preparing for XP Snapshot Operations