IBM Storwize V7000, Введение и руководство по внедрению

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Chapter 9. IBM Flex System V7000 Storage Node Copy Services
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With Change Volumes, a FlashCopy mapping exists between the primary volume and the
primary Change Volume. The mapping is updated during a cycling period (every 60 seconds
to one day.) The primary Change Volume is then replicated to the secondary Global Mirror
volume at the target site, which is then captured in another change volume on the target site.
This situation provides an always consistent image at the target site and protects your data
from being inconsistent during re synchronization.
Let us take a closer look at how Change Volumes might reduce replication traffic.
Figure 9-64 shows a number of I/Os on the source volume and the same number on the
target volume, and in the same order. Assuming that this set is the same set of data being
updated over and over, then these updates are wasted network traffic and the I/O can be
completed much more efficiently (Figure 9-65).
Figure 9-64 Global Mirror I/O replication without Change Volumes
In Figure 9-65, the same data is being updated repeatedly, so Change Volumes demonstrate
significant I/O transmission savings, because it is only necessary to send I/O number 16,
which was the last I/O before the cycling period.
Figure 9-65 Global Mirror I/O replication with Change Volumes
The cycling period can be adjusted by running chrcrelationship -cycleperiodseconds
<60-86400> . If a copy does not complete in the cycle period, the next cycle does not start until
the prior one has completed. It is for this reason that using Change Volumes gives you two
possibilities for RPO:
򐂰 If your replication completes in the cycling period, then your RPO is twice the
cycling period.
򐂰 If your replication does not complete within the cycling period, then your RPO is twice the
completion time. The next cycling period starts immediately after the prior one is finished.