The FlashCopy® feature
provides the semantics of a point-in-time copy by using an indirection
layer which intercepts I/Os that are targeted at both the source and
target virtual disks (VDisks).
Starting a FlashCopy mapping
causes this indirection layer to become active in the I/O path. This
occurs as an atomic command across all FlashCopy mappings that are in the consistency
group.
The indirection layer makes a determination about each
I/O. This determination is based upon the following criteria:
- The VDisk and LBA to which the I/O is addressed,
- Its direction (read or write)
- The state of an internal data structure, the FlashCopy bitmap.
The indirection layer either allows the I/O through to
the underlying storage, redirects the I/O from the target VDisk to
the source VDisk or stalls the I/O while it arranges for data to be
copied from the source VDisk to the target VDisk.
Table 1 provides
an overview of the FlashCopy I/O
path actions.
Table 1. FlashCopy I/O
path actions| VDisk |
Grain
already copied? |
Host I/O operations |
|---|
| Read |
Write |
|---|
| Source |
No |
Read from source |
Copies the grain to the most recently started
target VDisk for this source VDisk and then writes to the source VDisk. |
| Yes |
Read from source |
Write to source |
| Target |
No |
When the grain has not been copied, you can
use the following algorithm to determine the VDisk that is being read:- If newer target VDisks exist for the source VDisk of the FlashCopy mapping that is
the target VDisk being written to and the data has already been copied,
the read operation is from the target VDisk.
- If the source VDisk is not a target of another FlashCopy mapping, the read operation is
from the source VDisk.
- If newer target VDisks exist for the source VDisk of the FlashCopy mapping that is
the source VDisk being written to and the data has already been copied,
the read operation is from the target VDisk.
|
When the grain has not been copied or overwritten,
you can use the following algorithm:- Use the algorithm for the corresponding read to determine the
VDisk to read.
- If there is an older target VDisk and the data has not been copied
to this VDisk, the data is written to this VDisk.
- If there is a target VDisk for this VDisk and the data has not
been copied to this VDisk, the data is written to this VDisk.
- Writes to target.
|
| Yes |
Read from target When the grain has been copied,
you can use the following algorithm to determine the VDisk that is
being read:
- If newer target VDisks exist for this source VDisk and the grain
has already been copied, the read comes from the oldest target VDisk.
- If there are no newer target VDisks, the read comes from the source
VDisk.
|
Write to target - If newer target VDisks exist for this source VDisk and the grain
has already been copied, the read operation comes from the oldest
target VDisk. If there are no newer target VDisks, the read operation
comes from the source VDisk.
- If the grain has not already been copied to the next oldest target
VDisk for this source VDisk, the same data is also copied to the next
oldest target VDisk.
- Writes to target
|
Note: For cascaded FlashCopy operations,
a VDisk can be both the source and the target. When the VDisk is both
the source and target, the I/O path actions are handled as described
for a target VDisk.
Source read operations
Source
read operations are always passed through to the underlying source
VDisk.
Target read operations
To process
a read operation from the target VDisk, the FlashCopy mapping must consult the FlashCopy bitmap. If the data
has already been copied to the target VDisk, the read operation is
sent to the target VDisk. If the data has not already been copied,
the target read operation is either sent to the source VDisk, or to
another target VDisk if multiple target FlashCopy mappings exist for the source
VDisk. While the target read operation is outstanding, no write operations
that change the data that is being read are allowed to run.