Designing Disaster Recovery Clusters using Metroclusters and Continentalclusters, Reprinted October 2011 (5900-1881)

IMPORTANT: Metrocluster with Continuous Access EVA does not support asynchronous
replication on arrays prior to those that are listed above. For more information on
supported arrays, see the Disaster Tolerant Clusters Products Compatibility Feature Matrix
available at:
http://www.hp.com/go/hpux-serviceguard-docs —> High Availability.
◦ Synchronous mode: An I/O completion acknowledgement is sent to the host after data
is written to the source and destination caches.
• Suspension:
Suspend: When this command is enabled and failsafe mode is not enabled, I/O replication
is halted between the source and destination Vdisks. Source Vdisks continue to run I/O
locally and the I/O is also copied to a log Vdisk.
◦
◦ Resume: When this command is enabled, replication resumes between the source and
destination Vdisks. Merging of the log Vdisk or a full copy is also performed.
Log Disk
The DR group has storage allocated on demand called a log. The virtual log collects host write
commands and data if access to the destination storage system is severed. When a connection is
later re-established, the contents of the log are written to the destination Vdisk to synchronize it
with the source Vdisk. This process of writing the log contents, in the order that the writes occurred,
is called merging. Sometimes it is more practical to copy the source Vdisk directly to the destination
Vdisk. This copy operation is called a “full copy-all 1-MB”.
There is no manual method for forcing a full copy. It is an automatic process that occurs when a
log is full.
If synchronous replication is configured, a log can be in one of the following states:
• Normal: No source Vdisk is logging or merging.
• Logging: At least one source Vdisk is logging (capturing host write commands), but none are
merging.
• Merging: At least one source Vdisk is merging and logging.
If asynchronous replication is configured, a log can be in one of the following states:
• Normal: log disk operations are normal, and merging or a Full copy is in progress.
• Logging: At least one source Vdisk is logging data, but the data is not being merged. This
state is displayed if the link is down or the DR group is in the suspend mode.
• Merging: At least one source Vdisk is merging and logging.
NOTE: When the replication mode is manually changed from asynchronous to synchronous
mode, the state is displayed as Run Down.
When a DR group is in a logging state, the log will grow in proportion to the amount of write I/O
being sent to the source Vdisks. As the log grows, more space must be allocated out of the available
capacity of the disk group where it is a member. The capacity available to the log does not include
the spare capacity or any capacity being used by Vdisks, snapshots, or Snapclones. This means
the log disk will never overwrite any other data. Similarly, when a DR group is logging, the available
capacity for creating Vdisks, snapshots, and Snapclones does not include the capacity already
used by the log disk. Therefore, a log disk will never be overwritten by any other data.
214 Building Disaster Recovery Serviceguard Solutions Using Metrocluster with Continuous Access EVA