Building Disaster Recovery Serviceguard Solutions Using Metrocluster with Continuous Access EVA A.05.01

The replicating direction of a DR group is always from a source to a destination. In bidirectional
replication, an array can have both source and destination virtual disks that will reside in separate
DR groups. That is, one virtual disk cannot be both a source and destination simultaneously.
Bidirectional replication enables you to use both arrays for primary storage while they provide
disaster protection for another site.
The remote copy feature is intended not only for disaster recovery, but also to replicate data from
one storage system or physical site to another storage system or site. It also provides a method for
performing a backup at either the source or destination site.
HP P6000/EVA Continuous Access has the ability to suspend and resume replication. Some versions
of controller software support auto suspend when a full copy of the DR group is required. This
feature can be used to protect the data at the destination site by delaying the full copy operation
until a snapshot or snapclone of the data has been made. 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 to determine if your array
supports this feature.
IMPORTANT: Metrocluster with Continuous Access EVA does not support enabling auto suspend
on full copy feature.
Write modes
The remote replication write modes are as follows:
• Synchronous — The array acknowledges I/O completion after the data is cached on both the
local and destination arrays.
• Asynchronous — The array acknowledges I/O completion before data is replicated on the
destination array. Asynchronous write mode can be basic or enhanced, depending on the
software version of the controller.
◦ Basic Asynchronous mode — An I/O completion acknowledgement is sent to the host
immediately after data is written to the cache at the source controller, but before the data
is delivered to the cache on the destination controller. There is no requirement to wait for
the I/O completion acknowledgement from the destination controller.
◦ Enhanced Asynchronous mode — The host receives an I/O completion acknowledgement
after the data is successfully written to the disk based log/journal, which is used to queue
the writes, which occurs after the data is written to local cache. The asynchronous
replication process reads the I/O from the journal and replicates it, using current
methodologies, to destination EVA.
You specify the replication write mode when you create DR groups. The choice of write mode,
which is a business decision, has implications for bandwidth requirements and RPO. Synchronous
mode provides greater data currency because RPO will be zero. Asynchronous mode provides
faster response to server I/O, but at the risk of losing data queued at the source side if a site
disaster occurs. For complete information on which write modes are supported on each version of
controller software, see HP P6000 EVA Compatibility Reference available at http://www.hp.com/
support/manuals—>storage -> Storage Software -> Storage Replication Software -> HP P6000
Command View Software.
IMPORTANT: Metrocluster with Continuous Access EVA supports only Synchronous replication
mode and the Enhanced Asynchronous replication mode and does not support basic asynchronous
replication mode. 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—> HP Serviceguard Metrocluster with Continuous Access EVA.
Overview of EVA and HP P6000/EVA Continuous Access concepts 9