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

NOTE:
• A request to convert from synchronous mode to asynchronous mode is executed immediately.
A request to convert from asynchronous mode to synchronous mode is executed after the data
in the DR group write history log or the I/Os in the write pending queue are merged to the
destination array.
• During the conversion from enhanced asynchronous mode to synchronous mode, an I/O
throttling mechanism allows one new write to a DR group for every two DR group write history
log entry merges.
Asynchronous write mode
In asynchronous write mode, the source array acknowledges I/O completion after the data is
mirrored across both controllers on the source array. Asynchronous replication prioritizes response
time over data currency.
The asynchronous replication sequence depends on the version of controller software running on
the arrays. For more information, see the HP P6000 Enterprise Virtual Array Compatibility Reference.
XCS 10000000 supports the selection of either basic asynchronous mode or enhanced asynchronous
mode. The asynchronous write mode behavior for both basic and enhanced is unchanged from
earlier controller software. The user selectable asynchronous functionality is available only when
both the local and remote arrays are running XCS 10000000 or later.
Basic asynchronous mode
The basic asynchronous sequence is as follows:
1. A source array controller receives data from a host and stores it in cache.
2. The source array controller acknowledges I/O completion to the host.
3. The source array controller sends the data to the destination array controller.
4. The destination controller stores the data in cache.
5. The destination array controller mirrors data in its write cache and acknowledges I/O
completion to the source controller.
6. When the source array receives the acknowledgment from the target array, it removes the
data from the write history log.
The maximum size of the write pending queue limits asynchronous performance. VCS 3.xxx supports
64 outstanding host writes. All other VCS versions support 128 outstanding host writes. With a
small write pending queue, lower bandwidths struggle to support applications with erratic or high
peak load rates.
Enhanced asynchronous mode
XCS 6.xxx and later supports an enhanced asynchronous write mode, which uses the write history
log for the DR group as a place to hold I/Os waiting to be replicated to the remote array. You
can select the size of the log file up to a maximum of 2 TB.
NOTE: Enhanced asynchronous mode requires XCS 6.xxx or later on both arrays.
The enhanced asynchronous write sequence is as follows:
1. A source array controller receives data from a host and stores it in the cache of the local LUN
and the write history log cache.
2. The source array controller acknowledges I/O completion to the host.
3. The source controller takes data from the write history log and sends the data to the remote
array.
4. The destination array controller stores the data in cache and acknowledges I/O completion
to the source controller.
Choosing a write mode 21