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

Remote Copy Set Features
41Data Replication Manager HSG80 ACS Version 8.7P Configuration Guide
Write History Log Unit Performance Considerations
When the write history log unit is merging the captured write operations back to the target, the
host makes all I/O resources available to the write history log unit. This means that you can
expect at least a 90 percent reduction of host I/O capability for other operations. You must
then consider whether the host can afford to perform the merge during normal high-activity
hours, or whether the merge should take place during other nonactive times, such as late
evening.
A factor in this consideration is what other type of processing the host will be doing during the
merge. If the host cannot afford such a drastic reduction in capability, you may wish to
consider performing a full copy instead. During a full copy, the host will experience a less
drastic I/O performance loss of approximately 50%.
The write history log unit function is primarily intended for, and is most efficient with, short
duration outages. If a long duration outage is planned, the full copy option may be the best
solution.
Write History Log Unit Size Considerations
Choose the size of the write history log unit carefully. When the end of the write history log
unit is encountered, a full copy is initiated when the link is restored. You may wish to select a
write history log unit size that will accommodate a maximum outage of 8 hours duration, with
the merge occurring during the nonpeak hours that same day.
Display the write history log status by issuing the following CLI command:
SHOW REMOTE_COPY FULL
Switches
This section discusses the WRITE HISTORY LOG, ORDER_ALL, SUSPEND, and RESUME
switches.
Write History Log Unit Switches
The LOG_UNIT switch of the ADD ASSOCIATIONS command assigns a single, dedicated
write history log unit for an association set. This switch is valid only if all members of the
association set are in normal (not failsafe) error mode. Error mode is determined by the
ERROR_MODE switch of the SET RemoteCopySet command.
Note: When this command is entered, a header is immediately written to the write history log unit,
which may make it difficult or impossible to recover any user data previously written on the unit.
Take great care when you specify which unit should be the log unit
If NOLOG_UNIT is specified, the association set’s write history log unit is de-assigned.
NOLOG_UNIT is the default setting.
Note: A full copy occurs if you disable write history logging after logging operations have begun.
.