HP StorageWorks Data Replication Manager HSG80 ACS Version 8.7P Configuration Guide (AA-RPHZF-TE, March 2004)

Remote Copy Set Features
39Data Replication Manager HSG80 ACS Version 8.7P Configuration Guide
ADD ASSOCIATIONS Command
When you issue the ADD ASSOCIATIONS AssociationSetName
RemoteCopySetName command, it adds an association set with one member to the
controller pair’s configuration. Use this command on the node on which the initiator resides.
Issue the SET AssociationSetName ADD = RemoteCopySetName command to
add additional members.
Upon site failover, you must re-create the association sets and log units (see “Write History
Log Unit Switches” on page 41) at the target (failover) site, using the attributes that were set at
the initiator site.
Write History Logging
Write history logging means using a log unit to record a history of write commands and data
from the host. The write history log serves two purposes: to update the target via a mini-merge
when the ISL is temporarily broken and to update the initiator as part of the fast failback
procedure. In either case, a full copy is not required.
Mini-Merge: When the target becomes accessible again after a failure, a full copy is not
necessary. Only those host writes that were performed while the links were down are reissued.
This is referred to as a mini-merge.
Fast-Failback: During a planned failover, if write history logging has been enabled at the
target site, then when the failback is performed, the initiator site is synchronized through a
process called fast-failback. The writes are logged to the target site write history log. Then,
during a fast-failback, the initiator site is updated from the write history log.
If the target becomes inaccessible (because of a dual-target or dual-controller failure), the
writes that would have gone to the target are logged to the association set’s assigned write
history log unit. An inaccessible target in this context refers to both links or target controllers
going down.
Remote copy sets are marked for a full copy if they were actively performing write history
logging or mini-merging at the time of the controller failure. If a full copy was in progress at
the time of the disconnect, write history logging is not initiated and the full copy is restarted
when the target is accessible again.
Following a planned failover, if write history logging was enabled at the target site, the
initiator site is synchronized during the failback using fast-failback. In this scenario, writes
destined for the initiator during the failover period are logged to the association set’s write
history log unit. This means that only those writes issued since the failover occurred are
reissued; a full copy is not necessary.
Write History Log Units
Write history log units are assigned to an association set. Association sets are used by a host to
keep multiple units consistent with each other. The write history log unit must always fail over
(between dual-redundant controllers) with the remote copy sets assigned to the same
association set. All members of the association set must reside on the same controller and must
fail over both together and automatically. Upon site failover, the user must create a new write
history log unit at the target site.