Chapter 10. Copy services 575
The remote copy relationship type does not matter. (The remote copy relationship type can be
MM, GM, or GM with Change Volumes.) You can change the preferred node both to the
source and target volumes that are participating in the remote copy relationship.
10.7.14 Background copy performance
The background copy performance is subject to sufficient Redundant Array of Independent
Disks (RAID) controller bandwidth. Performance is also subject to other potential bottlenecks,
such as the intercluster fabric, and possible contention from host I/O for the IBM SAN Volume
Controller or IBM Storwize V5000 Gen2 system bandwidth resources.
Background copy I/O is scheduled to avoid bursts of activity that might have an adverse effect
on system behavior. An entire grain of tracks on one volume is processed at around the same
time but not as a single I/O. Double buffering is used to try to use sequential performance
within a grain. However, the next grain within the volume might not be scheduled for some
time. Multiple grains might be copied simultaneously, and might be enough to satisfy the
requested rate, unless the available resources cannot sustain the requested rate.
Global Mirror paces the rate at which background copy is performed by the appropriate
relationships. Background copy occurs on relationships that are in the InconsistentCopying
state with a status of Online.
The quota of background copy (configured on the intercluster link) is divided evenly between
all nodes that are performing background copy for one of the eligible relationships. This
allocation is made irrespective of the number of disks for which the node is responsible. Each
node in turn divides its allocation evenly between the multiple relationships that are
performing a background copy.
The default value of the background copy is 25 megabytes per second (MBps), per volume.
10.7.15 Thin-provisioned background copy
Metro Mirror and Global Mirror relationships preserve the space-efficiency of the master.
Conceptually, the background copy process detects a deallocated region of the master and
sends a special
zero buffer to the auxiliary.
If the auxiliary volume is thin-provisioned and the region is deallocated, the special buffer
prevents a write and, therefore, an allocation. If the auxiliary volume is not thin-provisioned or
the region in question is an allocated region of a thin-provisioned volume, a buffer of “real”
zeros is synthesized on the auxiliary and written as normal.
10.7.16 Methods of synchronization
This section describes two methods that can be used to establish a synchronized
relationship.
Important: The background copy value is a system-wide parameter that can be changed
dynamically but only on a per-system basis and not on a per-relationship basis. Therefore,
the copy rate of all relationships changes when this value is increased or decreased. In
systems with many remote copy relationships, increasing this value might affect overall
system or intercluster link performance. The background copy rate can be changed from
1 - 1000 MBps.
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