IBM Storwize V5030 User Manual

Chapter 10. Copy services 501
This copy-on-write process introduces significant latency into write operations. To isolate the
active application from this additional latency, the FlashCopy indirection layer is placed
logically between upper and lower cache. Therefore, the additional latency that is introduced
by the copy-on-write process is encountered only by the internal cache operations and not by
the application.
Figure 10-9 shows the logical placement of the FlashCopy indirection layer.
Figure 10-9 Logical placement of the FlashCopy indirection layer
Introduction of the two-level cache provides additional performance improvements to the
FlashCopy mechanism. Because now the FlashCopy layer is above lower cache in the IBM
Spectrum Virtualize software stack, it can benefit from read prefetching and coalescing writes
to backend storage. Also, preparing FlashCopy is much faster, because upper cache write
data does not have to go directly to backend storage but to lower cache layer.
Additionally, in the multitarget FlashCopy the target volumes of the same image share cache
data. This design is opposite to previous IBM Spectrum Virtualize code versions, where each
volume had its own copy of cached data.
10.3.9 FlashCopy and image mode volumes
FlashCopy can be used with image mode volumes. Because the source and target volumes
must be the same size, you must create a target volume with the same size as the image
mode volume when you are creating a FlashCopy mapping. To accomplish this task, use the
svcinfo lsvdisk -bytes volumeName command. The size in bytes is then used to create the
volume that is used in the FlashCopy mapping.
Target volume
Stage
Destage
Stage
Destage
Read
Write
Write Write
Read Read
Source volume
Upper cache
Lower cache
Indirection layer
FlashCopy bitmap
Copy from source to target

IBM Storwize V5030 Specifications

General

Brand	IBM
Model	Storwize V5030
Category	Storage
Language	English