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

to reduce the log size. When the log content has been drained, you can return the DR group to
enhanced asynchronous mode. Until the DR group is returned to enhanced asynchronous mode,
the DR group operates in synchronous mode, which may impact performance.
Allocated log file space is not decreased when DR group members are removed. Log space usage
will increase when members are being added to an existing DR group unless the size of the log
disk has reached the maximum of 2 TB or has been fixed to a user-defined value. For the default
maximum size, see the HP P6000 Enterprise Virtual Array Compatibility Reference.
NOTE:
• For XCS 6.0xx and 6.1xx, asynchronous replication mode is not available during the creation
of a DR group. When creating a DR group in either of these versions of XCS, you must wait
for the completion of the initial normalization to change the replication mode to asynchronous.
• For XCS 6.200 or later that are managed using HP P6000 Command View versions 8.0.1
or later, the creation of DR groups in asynchronous mode is allowed. These combinations of
XCS controller software and management software enable the addition or removal of DR
group members while in asynchronous replication mode. See the HP P6000 Enterprise Virtual
Array Compatibility Reference for details.
Normalization
The method of synchronizing source and destination virtual disks is called normalization. A
normalization can occur whenever the source and destination array need to be brought back into
synchronization. When a DR group is first created, a normalization occurs. If a DR group write
history log overflows, a normalization occurs to bring the source and destination arrays back into
synchronization.
When a DR group is first created, a full copy normalization occurs to copy all the data in the DR
group from the source array to the destination array, bringing the two arrays into synchronization.
A normalization can also occur if the write history log used by a DR group overflows or is invalidated
by the storage administrator. Normalizations copy data from the source array to the destination
array in 128 KB blocks. When a write history log overflows, the controller invalidates the log
contents and marks the DR group for normalization. In some cases, normalization will be optimized
to copy only blocks that were written before the write history log overflowed; not all the data in
the DR group.
DR group write history log size
You can set the maximum size for the DR group write history log while in synchronous mode. The
minimum size of the log depends on the replication mode. The default maximum value for the log
will differ for each replication mode, and is based on the controller software version. For details
on maximum and default log sizes, see the HP P6000 Enterprise Virtual Array Compatibility
Reference.
NOTE: If you are using XCS 6.000 or later and you choose enhanced asynchronous mode, the
same amount of space must be available for the DR group write history log at both the source and
destination sites when specifying the maximum log size.
XCS 6.000 and higher you can specify the size of the DR group write history log. It is important
to ensure that the write history log is large enough that, under normal operating circumstances, it
will not overflow and result in an unexpected normalization.
For XCS 6.2xx, and 09003000 or later, space for the log can be de-allocated by converting the
DR group to synchronous mode, waiting for the write history log to drain, and then specifying the
new size. Adding or removing members can be done in either synchronous or asynchronous mode
of operation.
Planning for DR group write history logs 43