Creating A Remote Mirror Pair; Performance Considerations - IBM System Storage DS3000 Programming Manual

Attention: Possible loss of data access – Any communication disruptions between the primary storage
subsystem and the secondary storage subsystem while resynchronization is underway could result in a
mix of new data and old data on the secondary logical drive. This condition would render the data
unusable in a disaster recovery situation.

Creating a Remote Mirror Pair

Before you create any mirror relationships, logical drives must exist at both the primary site and the
secondary site. The logical drive that resides on the local storage subsystem is the primary logical drive.
Similarly, the logical drive that resides on the remote storage subsystem is the secondary logical drive. If
neither the primary logical drive or the secondary logical drive exist, you must create these logical drives.
When you create the secondary logical drive, you must consider these items:
v The secondary logical drive must be of equal or greater size than the primary logical drive.
v The RAID level of the secondary logical drive does not have to be the same as the primary logical
drive.
Use these steps to create the logical drive.
1. Enable the Enhanced Remote Mirroring Feature.
2. Activate the Enhanced Remote Mirroring Feature.
3. Determine Candidates for a Remote Mirror Pair.
4. Create the Remote Mirror Relationship.

Performance Considerations

Consider these performance issues when you create mirror relationships:
v The controller owner of a primary logical drive performs a full synchronization in the background
while processing local I/O writes to the primary logical drive and associated remote writes to the
secondary logical drive. Because the full synchronization diverts controller processing resources from
I/O writes, full synchronization can have a performance impact to the host application.
v To reduce the performance impact, you can set the synchronization priority level to determine how the
controller owner will prioritize the full synchronization relative to other I/O activity. To set the
synchronization priority level, consider these guidelines:
– A full synchronization at the lowest synchronization priority level takes approximately eight times
as long as a full synchronization at the highest synchronization priority level.
– A full synchronization at the low synchronization priority level takes approximately six times as
long as a full synchronization at the highest synchronization priority level.
– A full synchronization at the medium synchronization priority level takes approximately
three-and-a-half times as long as a full synchronization at the highest synchronization priority level.
– A full synchronization at the high synchronization priority level takes approximately twice as long
as a full synchronization at the highest synchronization priority level.
v When the mirrored logical drive pair is in a Synchronization in Progress state, all host write data is
copied to the remote system. Both controller I/O bandwidth and I/O latency can affect host write
performance. Host read performance is not affected by the mirroring relationship.
v The time that it takes for data to be copied from the primary logical drive to the secondary logical
drive might impact overall performance. This impact is primarily caused by the delay and system
resource required for copying data to the remote mirror. Some delay might also occur because of the
limit to the number of simultaneous writes.
Chapter 8. About Enhanced Remote Mirroring premium feature
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