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The total amount of cache memory reserved for the FlashCopy bitmaps limits the amount of capacity that can be used as a FlashCopy target. Table 6-4 illustrates the relationship of bitmap space to FlashCopy address space, depending on the size of the grain and the kind of FlashCopy service being used. 

<img src='content_image/990978.jpg'>

Mapping consideration:  For multiple FlashCopy targets, you must consider the number of mappings. For example, for a mapping with a 256 KB grain size, 8 KB of memory allows one mapping between a 16 GB source volume and a 16 GB target volume. Alternatively, for a mapping with a 256 KB grain size, 8 KB of memory allows two mappings between one 8 GB source volume and two 8 GB target volumes. 

When you create a FlashCopy mapping, if you specify an I/O Group other than the I/O Group of the source volume, the memory accounting goes towards the specified I/O Group, not towards the I/O Group of the source volume. 

The default amount of memory for FlashCopy is 20 MB. This value can be increased or decreased by using the chiogrp  command. The maximum amount of memory that can be specified for FlashCopy is 2048 MB (512 MB for 32-bit systems). The maximum combined amount of memory across all copy services features is 2600 MB (552 MB for 32-bit systems). 

Bitmap allocation:  When creating a FlashCopy mapping, you can optionally specify the I/O group where the bitmap is allocated. If you specify an I/O Group other than the I/O Group of the source volume, the memory accounting goes towards the specified I/O Group, not towards the I/O Group of the source volume. This option can be useful when an I/O group is exhausting the memory that is allocated to the FlashCopy bitmaps and no more free memory is available in the I/O group. 

## FlashCopy general restrictions 

The following implementation restrictions apply to FlashCopy: 

GLYPH<SM590000> The size of source and target volumes in a FlashCopy mapping must be the same. 

GLYPH<SM590000> Multiple FlashCopy mappings that use the same target volume can be defined, but only one of these mappings can be started at a time. This limitation means that no multiple FlashCopy can be active to the same target volume. 

GLYPH<SM590000> Expansion or shrinking of volumes defined in a FlashCopy mapping is not allowed. To modify the size of a source or target volume, first remove the FlashCopy mapping. 

GLYPH<SM590000> In a cascading FlashCopy, the grain size of all the FlashCopy mappings that participate must be the same. 

GLYPH<SM590000> In a multi-target FlashCopy, the grain size of all the FlashCopy mappings that participate must be the same. 

