Building Disaster Recovery Serviceguard Solutions Using Metrocluster with Continuous Access EVA P6000 for Linux B.01.00.00

DR Group write history log
The DR group write history log is a virtual disk that stores a DR group's host write data. The log is
created when you create the DR group. Once the log is created, it cannot be moved.
In synchronous mode or basic asynchronous mode, the DR group write history log stores data
when replication to the destination DR group is stopped because the destination DR group is
unavailable or suspended. This process is called logging. When replication resumes, the contents
of the log are sent to the destination virtual disks in the DR group. This process of sending I/Os
contained in the write history log to the destination array is called merging. Because the data is
written to the destination in the order that it was written to the log, merging maintains an
I/O-consistent copy of the DR group's data at the destination.
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. You can set the maximum size for the
DR group write history log while in synchronous mode. 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.
In enhanced asynchronous mode, the DR group write history log acts as a buffer and stores the
data until it can be replicated. The consumption of the additional capacity required for the log
should not be viewed as missing capacity—it is capacity used to create the log.
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.
If necessary, you can reclaim allocated log disk space from a DR group in enhanced asynchronous
mode. You must first change the write mode to synchronous and then use the log control feature
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 details on maximum and
default log sizes for different replication modes, see the HP P6000 EVA Compatibility Reference
available at http://www.hp.com/support/manuals—> storage -> Storage Software -> Storage
Replication Software -> HP P6000 Command View Software.
When a write history log overflows, the controller invalidates the log contents and marks the DR
group for normalization to bring the source and destination arrays back into synchronization.
NOTE: When the replication mode is manually changed from asynchronous to synchronous
mode, the state is displayed as Run Down.
Failover
In HP P6000 Continuous Access replication, failover reverses replication direction for a DR group.
The destination array assumes the role of the source, and the source array assumes the role of the
destination. The process can be planned or unplanned. A planned failover allows an orderly
Overview of P6000/EVA and HP P6000 Continuous Access Concepts 7