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

Chapter 4 83
Online Backup
Snapshot File Systems
Snapshot File Systems
The VxFS file system provides a mechanism for taking snapshot images
of mounted file systems, which is useful for making backups.The
snapshot file system is an exact image of the original file system,
which is referred to as the snapped file system . The snapshot is a
consistent view of the file system “snapped” at the point in time the
snapshot is made. Selected files can be backed up from the snapshot
(using standard utilities such as cpio or cp) or the entire file system
image can be backed up (using the vxdump or fscat utilities).
The mount command is used to create a snapshot file system; there is no
mkfs step involved. A snapshot file system is always read-only and exists
only as long as it and the file system that has been snapped are mounted.
A snapped file system cannot be unmounted until any corresponding
snapshots are first unmounted. A snapshot file system ceases to exist
when unmounted. While it is possible to have multiple snapshots of a file
system made at different times, it is not possible to make a snapshot of a
snapshot.
This chapter describes the creation of snapshot file systems and gives
some examples of backing up all or part of a file system using the
snapshot mechanism.
Snapshot File System Disk Structure
A snapshot file system consists of:
• a super-block
• a bitmap
• a blockmap
• data blocks copied from the snapped file system
Figure 4-1, “The Snapshot Disk Structure,” shows the disk structure of a
snapshot file system.