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

Chapter 4 85
Online Backup
Snapshot File Systems
was made.
Initially, the snapshot file system satisfies read requests by simply
finding the data on the snapped file system and returning it to the
requesting process. When an inode update or a write changes the data in
block n of the snapped file system, the old data is first read and copied to
the snapshot before the snapped file system is updated. The bitmap
entry for block n is changed from 0 to 1 (indicating that the data for block
n can be found on the snapped file system). The blockmap entry for block
n is changed from 0 to the block number on the snapshot file system
containing the old data.
A subsequent read request for block n on the snapshot file system will be
satisfied by checking the bitmap entry for block n and reading the data
from the indicated block on the snapshot file system, rather than from
block n on the snapped file system. Subsequent writes to block n on the
snapped file system do not result in additional copies to the snapshot file
system, since the old data only needs to be saved once.
All updates to the snapped file system for inodes, directories, data in
files, extent maps, etc., are handled in this fashion so that the snapshot
can present a consistent view of all file system structures for the snapped
file system for the time when the snapshot was created. As data blocks
are changed on the snapped file system, the snapshot will gradually fill
with data copied from the snapped file system.
The amount of disk space required for the snapshot depends on the rate
of change of thesnapped file system and the amount of time the snapshot
is maintained. In the worst case, the snapped file system is completely
full and every file is removed and rewritten. The snapshot file system
would need enough blocks to hold a copy of every block on the snapped
file system, plus additional blocks for the data structures that make up
the snapshot file system. This is approximately 101 percent of the size of
the snapped file system. Normally, most file systems do not undergo
changes at this extreme rate. During periods of low activity, the snapshot
should only require two to six percent of the blocks of the snapped file
system. During periods of high activity, the snapshot might require 15
percent of the blocks of the snapped file system. These percentages tend
to be lower for larger file systems and higher for smaller ones.
NOTE If a snapshot file system runs out of space for changed data blocks, it is
disabled and all further access to it fails. This does not affect the snapped