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

Write history log size in synchronous or basic asynchronous mode
When using synchronous mode or basic asynchronous mode, if logging occurs because replication
has been suspended or the replication links have failed, the size of the log file expands in proportion
to the amount of writes. The size of the log file can increase only up to the user-specified maximum
value or to the controller's software default maximum value. The size of the log can't be changed
while in basic asynchronous mode. You must change the write mode to synchronous, change the
log file size, and then return to basic asynchronous mode.
In synchronous mode and basic asynchronous mode, the log grows as needed when the DR group
is logging and it shrinks as entries in the log are merged to the remote array. The controller considers
the log disk full when one of the following occurs:
• No free space remains in the disk group that contains the log disk.
• The log disk reaches 2 TB of Vraid1 space.
• The log reaches the default or user-specified maximum log disk size.
Write history log file size in enhanced asynchronous mode
The DR group write history log file size is set when you transition the DR group to enhanced
asynchronous mode. The space for the DR group write history log must be available on both the
source and destination arrays before the DR group is transitioned to enhanced asynchronous mode.
Once set, the space is reserved for the DR group write history log and cannot be reduced in size
without first transitioning to synchronous mode and allowing the log to drain. Note that running
in synchronous mode will probably have a large impact on server write I/O response times.
For known link outage periods, the size of the log required is directly related to the length of time
the link is down and the data generation rate. For planned periods of link down time, you can
calculate the log size required. Any write history log sizing done in anticipation of planned or
unplanned link outages will be in addition to the sizing done to ensure the log doesn't overflow
during normal operation.
Incorrect error message for minimum asynchronous replication log size
If you change the write mode of a DR group to enhanced asynchronous (between arrays running
XCS 6.0xx or later), you may see the following error message: Invalid user defined log
size.
If this occurs, check the size of the log. The log size must be a minimum of 1624 MB before you
can change the mode to enhanced asynchronous replication.
Log size displayed incorrectly when creating DR groups in a mixed controller software environment
When creating a DR group in an environment that includes arrays running different versions of
controller software, the requested and actual log size for the DR group may not match. This is due
to the mixed controller software environment, and also to the limited available space for the log.
This is a HP P6000 Command View display issue only and does not indicate a problem with the
DR group, which will function normally.
DR group write history log location
The array controller software chooses a default location for the DR group write history log if one
is not specified. The type of disks (online or near-online) used as the default location are determined
by the version of controller software. If you want to override the default, you can specify the location
of DR group. Table 4 (page 46) identifies the default process for selecting a disk group for the log.
Once the log is created, it cannot be moved.
Most arrays allow you to create DR group write history logs using economical near-online disks.
If the DR pair includes an array running VCS 3.xxx, the DR group write history log is automatically
created using a near-online disk group if one is available when the DR group is created. If the
array is running VCS 4.xxx, XCS 6.xxx, or XCS 09xxxxxx or later, you can override the automatic
44 Planning the array configuration