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

Remote Copy Set Features
36 Data Replication Manager HSG80 ACS Version 8.7P Configuration Guide
Synchronous
For the synchronous operation mode, the OUTSTANDING_IO setting refers to the number of
initiator-to-target writes that can be outstanding at any one time. If OUTSTANDING_IO is set
to 1 and the host issues four writes to a remote copy set, then only one write is in progress
between the initiator and target at a time. The other three writes are queued in the initiator
controller. As each write completes at the target, another write is issued from the initiator
controller write queue.
Asynchronous
For the asynchronous operation mode, the OUTSTANDING_IO setting applies to the number
of noncommitted host writes that can be outstanding at one time between the initiator and
target. Noncommitted means the write completion status has been returned to the initiator
host, but the write has not been completed at the target.
Suppose, for example, that the outstanding I/O is set to 5 and that the host issues a request,
waits for completion from the controller, then immediately issues another request. In
asynchronous mode, each request issued by the host is completed by the controller very
quickly. As a result, the host issues five requests before the remote site has completed the first
request. If the host then issues another (sixth) request, it exceeds the value of the outstanding
I/O.
When you exceed the outstanding I/O value, the system switches to synchronous mode. As
soon as an in-process I/O completes, another is started that attempts to empty the queue and
return to true asynchronous operation.
Outstanding Write Operations
Keep in mind that there is a controller-wide limit of 240 outstanding write operations, even if
the total number of writes 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, any new writes to the controller are queued
on the host.
High Outstanding I/O Values
Use caution when choosing an OUTSTANDING_IO setting, because writes to the targets are
handled on a FIFO (first in, first out) basis. 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.
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 noncommitted writes in the
event of an initiator site disaster.
Suspend and Resume Switches
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.