Managing Systems and Workgroups: A Guide for HP-UX System Administrators

Planning a Workgroup
Planning to Manage File Systems
Chapter 2 99
Typically, the system administrator will create a new snapshot after
correcting the problem (for example, by using a larger snapshot device,
or by choosing a time when the primary file system is less volatile).
How does an OnLineJFS backup differ from a standard backup?
An OnLineJFS backup involves using a snapshot of the file system,
rather than the file system itself.
Explicit information on how to perform an online backup can be found at
“Backing Up a JFS Snapshot File System” on page 694.
For purposes of online backups, what are the advantages and
disadvantages of snapshots compared to using the LVM lvsplit utility?
This question assumes you have installed both HP MirrorDisk/UX and
HP OnLineJFS.
Advantages of using lvsplit:
• You can do the backup using a read-only volume group.
• You can use fbackup, which is not supported for JFS snapshot file
systems.
• lvsplit works atomically on several logical volumes at once;
whereas it is not possible to take a snapshot of more than one file
system at a time.
• If a disk fails, mirroring provides more protection. (You can, however,
take a snapshot of a mirrored volume; the snapshot itself need not be
mirrored.)
• lvsplit might provide better performance, since blocks being
written are copied to the snapshot volume, thereby increasing disk
I/O. However, lvmerge will also increase disk I/O, and an fsck will
be necessary also.
Advantages of JFS snapshot:
• Snapshots require less disk space than do file system mirror images.
• Snapshots do not require an fsck, which is necessary after executing
an lvsplit.
• Snapshots are more foolproof procedure: executing lvmerge with an
incorrect argument sequence can destroy the disk blocks created
after lvsplit.
Does JFS have an interface to a snapshot file system?