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

Data Replication Manager Concepts 2–5
Outstanding Write Operations
Users should be aware that there is a controller-wide limit of 240 outstanding write
operations even if the total is greater than 240. For example, you might have 12
synchronous remote copy sets each with a value of 100. The maximum outstanding writes
are 240 and not 1200. When 240 outstanding writes are reached, then any new writes to
the controller are queued.
High Outstanding I/O Values
Use caution when choosing an OUTSTANDING_IO setting since writes to the targets are
handled in a FIFO (First In, First Out) manner. As a result, remote copy sets with higher
OUTSTANDING_IO values could potentially starve other remote copy sets if the write
rates become very high at any one time.
Low Outstanding I/O Values
On the other hand, choosing a lower setting may starve a very active remote copy set. In
the case of asynchronous remote copy sets, a lower OUTSTANDING_IO value may be
appropriate. This lower value limits the number of outstanding non-committed writes in
the event of an initiator site disaster.
Suspend/Resume
The SUSPEND switch suspends the update to the remote copy set target and starts the
write history logging of write commands and data from the unit.
NOTE: This switch is valid only in normal error mode (not failsafe).
The RESUME switch initiates the mini-merge restore of the specified remote target unit.
This switch enables the initiator to read the log unit and send the write commands, in
order, to the target, which brings the target into congruency with the initiator. For more
information on mini-merge, see the “Write History Logging” section in this chapter.
NOTE: The SET AssociationSetName NOLOG_UNIT command terminates any suspended
targets that are currently active.