Data Pools - Hitachi AMS2100 User Manual

Data pools

1-4
Hitachi AMS 2000 Family Copy-on-Write SnapShot User Guide
To maintain the snapshot image of the P-VOL when new data is
written to the P-VOL, SnapShot copies data that is being replaced to
the data pool. V-VOL pointers in cache memory are updated to
reference the original data's new location in the data pool.
2 illustrates SnapShot volumes and data pool interaction.
A V-VOL provides a virtual image of the P-VOL at the time of the
snapshot. Unlike the P-VOL, which contains actual data, the V-VOL is
made up of pointers to the data in the P-VOL, and to original data
that has been changed in the P-VOL since the last snapshot and
which has been copied to the data pool.
V-VOL's are set up with LUs that are the same size as the related P-
VOL. This capacity is not actually used and remains available as free
storage capacity. This V-VOL sizing requirement (must be equal to
the P-VOL), is necessary for SnapShot and array logic. Also,
V-VOL pointers to data in the data pool and P-VOL actually reside in
cache memory. Because of this, part of your array's cache is reserved
for SnapShot when it is enabled. (See
on page 2-31 and Appendix D, Using SnapShot with Cache Partition
Manager.)
The data pool holds data from the P-VOL that is being replaced.
After a snapshot is taken, the V-VOL maintains pointers to P-VOL
data. If changes occur, before the updated block is written to the P-
VOL, the data that is being replaced is first copied to the data pool.
The V-VOL pointer to this block is updated to the new address in the
data pool. Thus, the V-VOL maintains the point-in-time image of the
P-VOL, until the next snapshot is taken.
A data pool can be shared by multiple SnapShot pairs.
The data pool's function in the SnapShot process is illustrated in
Figure 1-2.
SnapShot overview
Maximum supported capacity
Figure 1-