Data Pools; Guaranteed Write Order And The Update Cycle - Hitachi AMS 2000 User Manual

Data pools

Data from the host is continually updated to the P-VOL, as it occurs. The
data pool on the local side stores the changed data that accumulates before
the next the update cycle. The local data pool is used to update the S-VOL.
Data that accumulates in the data pool is referred to as differential data
because it contains the difference data between the P-VOL and S-VOL.
The data in the S-VOL following an update is complete, consistent, and
usable data. When the next update is to begin, this consistent data is copied
to the remote data pool. This data pool is used to maintain previous point-
in-time copies of the S-VOL, which are used in the event of failback.

Guaranteed write order and the update cycle

S-VOL data must have the same order in which the host updates the P-VOL.
When write order is guaranteed, the S-VOL has data consistency with the
P-VOL.
As explained in the previous section, data is copied from the P-VOL and local
data pool to the S-VOL following the update cycle. When the update is
complete, S-VOL data is identical to P-VOL data at the end of the cycle.
Since the P-VOL continues to be updated while and after the S-VOL is being
updated, S-VOL data and P-VOL data are not identical.
However, the S-VOL and P-VOL can be made identical when the pair is split.
During this operation, all differential data in the local data pool is
transferred to the S-VOL, as well as all cached data in host memory. This
cached data is flushed to the P-VOL, then transferred to the S-VOL as part
of the split operation, thus ensuring that the two are identical.
If a failure occurs during an update cycle, the data in the update is
inconsistent. Write order in the S-VOL is nevertheless guaranteed — at the
point-in-time of the previous update cycle, which is stored in the remote
data pool.
Figure 1-2
of P-VOL data.
1–4
Hitachi AMS 2000 Family TrueCopy Extended Distance User Guide
shows how S-VOL data is maintained at one update cycle back
Overview