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114 IBM System Storage DS6000 Series: Copy Services with IBM System z
10.2 FlashCopy establish phase performance
The FlashCopy of a volume has two distinct periods:
The
initial logical FlashCopy (also called establish)
The
physical FlashCopy (also called background copy)
The FlashCopy establish phase, or
logical FlashCopy, is the period of time when the
microcode is preparing things, such as the bitmaps, necessary to create the FlashCopy
relationship so the microcode can properly process subsequent reads and writes to the
related volumes. During this logical FlashCopy period, no writes are allowed to the volumes.
After the logical relationship has been established, normal I/O activity is allowed to both
source and target volumes according to the options selected.
When there are a large number of volumes, the method used to invoke the FlashCopy
commands can influence the time it takes to complete the logical FlashCopy for all FlashCopy
pairs. An in-band invocation will source the commands to the microcode faster than an
out-of-band method in most cases. An in-band establish is one that uses the CCW
commands directly, like in z/OS.
The fastest out-of-band method is the DS CLI. The method to control or invoke FlashCopy
should be selected based upon the total solution requirements and not strictly on
logical
FlashCopy performance considerations, unless this is a critical performance issue.
Additionally, there is a modest performance impact to logical FlashCopy performance when
using incremental FlashCopy. In the case of incremental FlashCopy, the DS6000 must create
additional metadata (bitmaps). However, the impact is negligible in most cases.
Finally, the placement of the FlashCopy source and target volumes has an effect on the
establish performance. You can refer to the previous section for a discussion of this topic as
well as to Table 10-1 on page 113 for a summary of the recommendations.
10.3 Background copy performance
The background copy phase, or physical FlashCopy, is the actual movement of data from the
source volume to the target volume. If the FlashCopy relationship was established requesting
the NOCOPY option, then only write updates to the source volume will force a copy from the
source to the target. This forced copy is also called a
collision.
If the COPY option was selected, then upon completion of the logical FlashCopy establish
phase, the source will be copied to the target in an expedient manner.
If a large number of volumes have been established, then do not expect to see all pairs
actively copying data as soon as their logical FlashCopy relationship is completed. The
DS6000 microcode has algorithms that limit the number of active pairs copying data. These
algorithms will try to balance active copy pairs across the DS6000 device adapter resources.
Additionally, they will limit the number of active pairs such that there remains bandwidth for
host or server I/Os.
Note: The term collision describes a forced copy from the source to the target because a
write to the source has occurred. This occurs on the
first write to a track. Note that since
the DS6000 writes to non-volatile cache, there is typically no direct response time delay on
host writes. The forced copy only occurs when the write is destaged onto disk.