HP P6000 Continuous Access Implementation Guide (T3680-96431, August 2012)

9 Managing remote replication
This chapter describes routine and advanced procedures for managing remote replication.
Using remote replication in a mixed array environment
The remote replication features that are available depend on the controller software version. The
following rules apply to remote replication between an array with XCS controller software and an
array with VCS controller software:
• If a feature such as the supported number of virtual disks in a DR group is supported differently
in the source and destination arrays, the more restrictive value applies to the source-destination
pair.
• If a feature such as enhanced asynchronous write mode is not available in either array, it
cannot be used in a source-destination pair.
For version-specific features and current support limits, see the HP P6000 Enterprise Virtual Array
Compatibility Reference.
Managing merges and normalization
When logging occurs in synchronous write mode or basic asynchronous write mode, there is
disparity between data being processed at the local site and data at the remote site. A merge or
normalization corrects this disparity when the cause for the interruption is remedied.
• A merge sends data from the log disk in write order so the destination copy remains crash
consistent.
• A normalization copies data in 1-MB chunks from the source virtual disk to the destination
virtual disk, and data is inconsistent until the normalization is complete.
Throttling a merge I/O after logging
Throttling is the channeling of I/O to merge critical data first by suspending and then resuming
specific DR groups.
When replication has been halted, data is logged on the source array (except when failsafe is
enabled). When replication is resumed, if there are several DR groups with large logs, they may
compete for bandwidth if they try to merge simultaneously. By resuming critical DR groups first,
the controllers merge only the most critical data first, allowing this data to be synchronized and
become accessible before the less important DR group data. As the more important DR groups
finish merging, you can resume the replication of DR groups that were left suspended.
Maintaining I/O performance while merging
During a merge, the controller software manages merge writes and host I/O, allowing one new
host I/O into the write history log for every two I/Os merged out of the write history log to the
destination array. This ratio ensures that the log empties faster than it fills. If host performance is
an issue during merging, you can postpone merging to a period of low demand by using the
suspend command to prevent merging and then using the resume command to allow merging.
VCS 6.0 or later uses this same ratio to empty the log when it receives a request to change the
write mode from enhanced asynchronous to synchronous.
Preparing for a normalization
IMPORTANT: An HP P6000 Business Copy license is required for this procedure.
When a DR group write history log is marked for a normalization, there is a risk to the destination
copy once the process begins. You can eliminate this risk by using the HP P6000 Business Copy
124 Managing remote replication