HP Application Recovery Manager software A.06.10 Concepts guide (March 2008)

3. The application host is prepared, bringing data into a consistent state. For online
backup, the database is quiesced. For offline backup, the database is brought
offline. If the ZDB option Dismount application system is set ON, the filesystems
involved are dismounted.
Freezing operation of the application or database
While a replica is being created, operation of the application or section of the
database concerned must be frozen.
The Application Integration Agent puts the application database or filesystem into
the required state. This could be with all database updates stopped for an offline
replication, or with all database updates re-routed to log files in the case of an online
replication:
• In offline replication, the database is taken offline, so that all file I/O is stopped
while the replica is created. The database is usually placed into a consistent state,
for instance by applying any previously unapplied redo logs.
Although creating a replica is very fast, the application is offline for a short time,
so this method is less suitable for high availability applications.
• In online replication, the database is placed into hot-backup mode while the
replica is created. In this mode, the database remains online, but all database
I/O is diverted to transaction log files instead of updating the database. After
the replica is made, the transaction log files are applied to the database to bring
it up-to-date.
This method of replication reduces impact on the application to a minimum,
making it suitable for uninterrupted operations.
The steps concerned in these operations can be controlled automatically when backing
up database applications supported by Application Recovery Manager. However,
it is also possible to set up similar behavior when backing up other applications or
filesystems; pre- and post-exec options enable you to specify scripts to run before
and after replication.
In both cases, the effect of the backup process on the application is limited to the
period during which the replica is created. In the “online” case, database operation
is never stopped (zero downtime) and the effect on performance is minimal, limited
mainly to the effect of having to write increased information to the transaction logs.
Both online and offline backup are also available with other tape based products.
However, there is a much greater impact on application/database operation since
with conventional backup to tape, a database has to be put into hot-backup mode
or taken offline for the whole of a backup session.
ZDB session process52