Chapter 10. Copy services 497
10.3.5 Grains and the FlashCopy bitmap
When data is copied between volumes, it is copied in units of address space that are known
as
grains. Grains are units of data that are grouped to optimize the use of the bitmap that
tracks changes to the data between the source and target volume. You can use 64 kibibytes
(KiB) or 256 KiB grain sizes (256 KiB is the default). The FlashCopy bitmap contains 1 bit for
each grain, and is used to show whether the source grain was copied to the target. The 64
KiB grain size uses bitmap space at a rate of four times the default 256 KiB size.
The FlashCopy bitmap dictates read and write behavior for the source and target volumes.
Source reads
Reads are performed from the source volume, which is the same as for non-FlashCopy
volumes.
Source writes
Writes to the source cause one of the following actions:
If the grain was not copied to the target yet, the grain is copied before the actual write is
performed to the source. The bitmap is updated to indicate that this grain is already copied
to the target.
If the grain was already copied, the write is performed to the source as usual.
Target reads
Reads are performed from the target if the grain was copied. Otherwise, the read is
performed from the source and no copy is performed.
Target writes
Writes to the target cause one of the following actions:
If the grain was not copied from the source to the target, the grain is copied from the
source to the target before the actual write is performed to the source. The bitmap is
updated to indicate that this grain is already copied to the target.
If the entire grain is being updated on the target, the target is marked as split with the
source (if there is no I/O error during the write) and the write goes directly to the target.
If the grain in question was already copied from the source to the target, the write goes
directly to the target.
The FlashCopy indirection layer algorithm
Imagine the FlashCopy indirection layer as the I/O traffic director when a FlashCopy mapping
is active. The I/O is intercepted and handled according to whether it is directed at the source
volume or at the target volume, depending on the nature of the I/O (read or write) and the
state of the grain (whether it was copied).
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