content_image/986029.jpg
## FlashCopy indirection layer 

The FlashCopy indirection layer governs the I/O to the source and target volumes when a FlashCopy mapping is started, which is done by using the FlashCopy bitmap. The purpose of the FlashCopy indirection layer is to enable the source and target volumes for read and write I/O immediately after the FlashCopy is started. 

The indirection Layer intercepts any I/O coming from a host (read or write operation) and addressed to a FlashCopy volume (source or target). It determines whether the addressed volume is a source or a target, its direction (read or write), and the state of the bitmap table for the FlashCopy mapping that the addressed volume is in. It then decides what operation to perform. The different I/O indirections are described next. 

## Read from the source Volume 

When a user performs a read operation on the source volume, there is no redirection. The operation is similar to what is done with a volume that is not part of a FlashCopy mapping. 

## Write on the source Volume 

Performing a write operation on the source volume modifies a block or a set of blocks, which modifies a grain on the source. It generates one of the following actions, depending on the state of the grain to be modified. Consider the following points: 

GLYPH<SM590000> If the bitmap indicates that the grain was copied, the source grain is changed and the target volume and the bitmap table remain unchanged, as shown in Figure 11-5. 

<img src='content_image/986029.jpg'>

