96 Understanding the IBM TotalStorage Open Software Family
򐂰 Minimum granularity is the entire vDisk.
򐂰 The source and target must be the same size.
򐂰 There is no multiple relationship support; therefore, there is only one
FlashCopy per volume.
򐂰 There is no incremental support.
򐂰 An existing FlashCopy relationship can be stopped.
򐂰 A FlashCopy relationship is persistent until it is explicitly unmapped.
򐂰 The size of source and target virtual volumes cannot be altered (increased or
decreased) during the mapping.
򐂰 There is no limit on the number of virtual disks that are supported. However,
the maximum quantity of source virtual disk per I/O group is 16 TB.
3.2.8 Peer-to-Peer Remote Copy
This section looks at the principles of PPRC.
How it works
The general application of Peer-to-Peer Remote Copy seeks to maintain two
copies of a data set. Often, the two copies are separated by some distance,
hence the term
remote, but this is not required.
The SVC assumes that the Fibre Channel (FC) fabric to which it is attached
contains hardware, which achieves the long distance requirement for the
application. This hardware makes storage “at a distance” accessible as though it
were local storage. Specifically, it enables two SVC clusters to connect (FC
login) to each other and establishes communications in the same way as though
they were located nearby on the same fabric. The only difference is in the
expected latency of that communication, the bandwidth capability of the link, and
the availability of the link as compared with the local fabric.
The relationship between the two copies is not symmetric. One copy of the data
set is considered the primary copy (sometimes also known as the
source copy).
This copy provides the reference for normal run-time operation. Updates to this
copy are shadowed to a secondary copy (sometimes known as the
destination
copy or target copy). The secondary copy is not normally referenced for
performing I/O.
If the primary copy fails, the secondary copy can be enabled for I/O operation. A
typical use of this function may involve two sites where the first provides service
during normal running and the second is only activated when a failure of the first
site is detected.

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