Dell PowerVault MD3600f/MD3620f Remote Replication Functional Guide
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Database Consistency and Database Hot Backups
After suspending AWOC for database virtual disk replication, and, if a disaster occurs while resuming
RR then the remote database virtual disks are not recoverable. This is due to the delta log processing
of missing I/O in non-original write order. Once RR’s resume operations are complete, the virtual disk
pairs are once again in synchronization, and the remote database virtual disks are recoverable in
disaster recovery processing.
In order to have usable database virtual disks in the event that a disaster occurs in the middle of a
resume operation, it is important to snapshot the remote database virtual disks at the very instant that
they are suspended and before the replication is resumed and uses the delta log to re-synchronize.
This requires the auto-resume feature to be turned off and a snapshot is taken at the time of
suspension.
Once the resume operation completes, a second snapshot is taken. This provides the ability to recover
from a primary site disaster. If a disaster occurs during resume operations, the first snapshot is used for
recovery. Once the resume operation completes, the second snapshot is used for recovery.
In both cases, either snapshot will be database consistent and u sable for database recovery in the
event of a disaster. We can achieve this using Database Hot Backups.
All databases provide a hot backup operation that allows a storage array to snapshot the database
virtual disks to create a point-in-time copy for backup and/or recovery purposes.
Database hot backups perform a number of functions that ensure recoverability using snapshots or
copies of the database virtual disks. First, the database internal data (pointers, buffers, tablespace data,
and recovery information) are all persisted to the database virtu al disks. Enough information is
therefore stored to ensure a successful recovery using the database virtual disks including the
important logs.
Disaster Recovery using RR will use this same hot backup operation to create a remote snapshot of
the database virtual disks. In essence, the remote snapshot is identical to the local snapshot.
During this hot backup window we will use RR’s suspend feature to flush the data written on the
primary site virtual disks to their replication counter parts on the remote system; we can then take a
snapshot of the remote virtual disks to create a recoverable set of database virtual disks. RR will then
resume and the database will exit hot backup mode.
NOTE:
Periodically performed, this process will create recovery snapshots (also called Recovery
Points) which can be used as remote backup images, or to rep rovision the database itself (a second
copy residing at the remote site), and in the event of a disaster and the current set of virtual disks are
not recoverable, the last recovery snapshot will be used instead