Choosing the Right Disk Technology in a High Availability Environment DRAFT Version 2.0, August 1996

DRAFT -- Revision 2.0
August 22, 1996Page 49
on-line backup. Consistency refers to the ability of the backup to provide a self-
contained copy of all of the data. Some Relational Database systems provide an
on-line consistent backup facility either as a tool, or as a procedure using OS
backup tools.
Off-line backups can be performed by first shutting down the application and
then doing the backup. During the time that the backup occurs, access to the
data is not allowed.
Alternatively, splitting off one side of mirrored disks and then doing the backup
allows the application to continue to access the data while the backup is
occurring . This is an on-line backup based on a snapshot of the data and can be
done on the same system or a different system from the one where the
application is running. The offline mirror is then merged back into the online
mirror after the backup has completed.
The HP-UX version 10.X feature called read-only volume groups is required to
perform the on-line backup on a different system. An on-line concurrent backup
is normally inconsistent unless the application is first shutdown. The procedure
for ensuring that the backup is consistent is:
1. Quiesce the application for data consistency.
2. Perform an lvsplit operation to split off one side of the mirror.
3. Resume the application.
4. Optionally, activate the volume group as read-only on a different
system.
5. Do the backup.
6. Deactivate the volume group if it was activated on another
system.
7. Perform an lvmerge operation to return the offline mirror to the
mirrored set.
For those applications that use a file system, the optional OnLine JFS product
also provides the capability to do an online backup. A snapshot file system is
created to record the original data that is being changed while the application is
online during the backup. The backup accesses the original data by referring to
the snapshot file system rather than the original file system.
Total capacity requirements
The total amount of disk space required may force a choice of disk technology.
Disk arrays provide the largest total capacity on a system, so it might be
necessary to choose that technology just to get the needed capacity.