Indirection layer The  FlashCopy indirection layer  governs the I/O to the source and target volumes when a  FlashCopy mapping is started. This process is done by using a FlashCopy bitmap. The  purpose of the FlashCopy indirection layer is to enable both the source and target volumes for  read and write I/O immediately after FlashCopy starts. The following description illustrates how the FlashCopy indirection layer works when a  FlashCopy mapping is prepared and then started. When a FlashCopy mapping is prepared and started, the following sequence is applied: FlashCopy provides the semantics of a point-in-time copy that uses the indirection layer,  which intercepts I/O that is directed at either the source or target volumes. The act of starting  a FlashCopy mapping causes this indirection layer to become active in the I/O path, which  occurs automatically across all FlashCopy mappings in the Consistency Group. The  indirection layer then determines how each of the I/O is to be routed based on the following  factors: The indirection layer allows the I/O to go through the underlying volume. It redirects the I/O  from the target volume to the source volume, or queues the I/O while it arranges for data to be  copied from the source volume to the target volume. The process of queueing the write  operations on the source volume while the indirection layer completes the grain copy on the  target volume is called  copy-on-write . Table 6-2 summarizes the indirection layer algorithm. 1. Flush the write cache to the source volume or volumes that are part of a Consistency  Group. 2. Put the cache into write-through mode on the source volumes. 3. Discard the cache for the target volumes. 4. Establish a sync point on all of the source volumes in the Consistency Group (creating the  FlashCopy bitmap). 5. Ensure that the indirection layer governs all of the I/O to the source volumes and target. 6. Enable the cache on source volumes and target volumes. GLYPH<SM590000> The volume and the logical block address (LBA) to which the I/O is addressed GLYPH<SM590000> Its direction (read or write) GLYPH<SM590000> The state of an internal data structure, the FlashCopy bitmap Table 6-2 Summary table of the FlashCopy indirection layer algorithm Volume  being  accessed Has the grain  been copied?  Host I/O operation Read Write Source No Read from the source  volume. Copy grain to the most  recently started target for this  source, then write to the  source. Yes Read from the source  volume. Write to the source volume. 152  IBM FlashSystem 9200 and 9100 Best Practices and Performance Guidelines