Compaq StorageWorks Data Replication Manager HSG80 ACS Version 8.5P Operations Guide

Data Replication Manager Concepts 2–9
Log Unit Restrictions
■ Up to 12 log units can be assigned (12 possible remote copy sets).
■ There can be only one log unit assigned to an association set.
■ Log units must be either a mirrorset or striped mirrorset.
■ Access must be disabled.
■ Write-back caching must be disabled.
■ Other unit settings must be the default settings.
■ Log unit must reside at the current initiator site.
■ Upon site failover the log unit and association set must be reconfigured.
■ Log unit cannot be a partitioned unit.
■ Log unit is a fixed size.
Choose the size of the log unit carefully. When the end of the log unit is encountered, a
fully copy is initiated when the link is restored. The amount of time before hitting the end
of a log unit depends upon how long the links are down, how long a target backup takes,
host write workload, the size of the log unit, and the number of remote copy sets actively
logging to the same log unit. Display the log status by using the following CLI command:
SHOW REMOTE_COPY FULL
Log Unit
The LOG_UNIT switch assigns a single, dedicated log unit for this association set.
NOTE: 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.
IMPORTANT: When the command is entered, a header is immediately written to the log unit,
which may make it difficult or impossible to recover any user data previously written on the unit.
Care should be taken in specifying which unit should be the log unit.
If NOLOG_UNIT is specified, the association set’s log unit is deassigned.
NOLOG_UNIT is the default setting.
NOTE: You may incur a full copy if you disable write history logging after logging operations
have begun.