IBM DS6000 Manual Do Utilizador

IBM System Storage DS6000 Series: Copy Services with IBM System z

The FlashCopy of a volume has two distinct periods:

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The 

logical FlashCopy (also called 

)

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The 

 FlashCopy (also called 

 copy)

The FlashCopy establish phase, or 

 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 

 

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.

The 

 phase, or 

 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 

. 

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 

describes a forced copy from the source to the target because a 

write to the source has occurred. This occurs on the 

 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.