Chapter 6. Volume configuration 343
This migration process requires more user interaction, but it offers some benefits, for
example, if you migrate a volume from a tier 1 storage pool to a lower performance tier 2
storage pool. In step 1, you create the copy on the tier 2 pool. All reads are still performed in
the tier 1 pool to the primary copy. After the synchronization, all writes are destaged to both
pools, but the reads are still only done from the primary copy.
Now you can switch the role of the copies online (step 3), and test the performance of the new
pool. If you are done testing your lower performance pool, you can split or delete the old copy
in tier 1, or switch back to tier 1 in seconds, in case tier 2 pool did not meet your performance
requirements.
6.10 I/O throttling
You can set a limit on the number of I/O operations that are accepted for a volume. The limit is
set in terms of I/O operations per second (IOPS) or MBps. By default, no I/O throttling rate is
set when a volume is created.I/O throttling is also referred to as I/O governing.
Base the choice between I/O and MB as the I/O governing throttle on the disk access profile
of the application. Database applications generally issue large amounts of I/O, but they
transfer only a relatively small amount of data. In this case, setting an I/O governing throttle
that is based on MBps does not achieve much. It is better to use an IOPS as a second
throttle.
At the other extreme, a streaming video application generally issues a small amount of I/O,
but it transfers large amounts of data. In contrast to the database example, setting an I/O
governing throttle that is based on IOPS does not achieve much, so it is better to use an
MBps throttle.
An I/O governing rate of 0 does not mean that zero IOPS (or MBps) can be achieved. It
means that no throttle is set.
Note:
I/O governing does not affect FlashCopy and data migration I/O rates.
I/O governing on a Metro Mirror and Global Mirror secondary volume does not affect the
rate of data copy from the primary volume.
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