HP JFS 3.3 and HP OnLineJFS 3.3 VERITAS File System 3.3 System Administrator's Guide

Chapter 4 87

Online Backup

Using a Snapshot File System for Backup

Once a snapshot ﬁle system is created, it can be used to perform a

consistent backup of the snapped ﬁle system. Backup programs that

function using the standard ﬁle system tree (such as cpio) can be used

without modiﬁcation on a snapshot ﬁle system, since the snapshot

presents the same data as the snapped ﬁle system. Backup programs

that access the disk structures of a VxFS ﬁle system need to be

snapshot-aware to work correctly with a snapshot ﬁle system. The VxFS

utilities understand snapshot ﬁle systems and make suitable

modiﬁcations in their behavior so that their operation on a snapshot ﬁle

system is indistinguishable from that on a normal ﬁle system.

Other backup programs that normally read the raw disk image cannot

work on snapshots without modiﬁcation. These programs can use the

fscat command to obtain a raw image of the entire ﬁle system identical

to that which would have been obtained by a dd of the disk device

containing the snapped ﬁle system at the exact moment the snapshot

was created. The snapread ioctl takes arguments similar to those of the

read system call and returns the same results as would have been

obtained by performing a read on the disk device containing the snapped

ﬁle system at the exact time the snapshot was created. In both cases,

however, the snapshot ﬁle system provides a consistent image of the

snapped ﬁle system with all activity complete; it is an instantaneous

read of the entire ﬁle system. This is a marked contrast to the results

that would be obtained by a dd or read of the disk device of an active ﬁle

system.

If a complete backup of a snapshot ﬁle system is made through a utility

such as vxdump and is later restored, it will be necessary to fsck the

restored ﬁle system because the snapshot ﬁle system is only consistent

and not clean. The ﬁle system may have some extended inode operations

that must be completed, though there should be no other changes. Since

the snapshot ﬁle system is not writable, it cannot be fully checked.

However, the fsck -n command can be used to report any

inconsistencies.

Creating a Snapshot File System

A snapshot ﬁle system is created by using the -o snapof= option of the

mount command. The -o snapsize= option may also be required if the