Veritas™ File System 5.0.1 Administrator's Guide

data for block n can be found on the snapshot 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, instead of from block n on the snapped file
system. This technique is called copy-on-write. 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, and so forth, are handled in this fashion so that the snapshot can present
a consistent view of all file system structures on the snapped file system for the
time when the snapshot was created. As data blocks are changed on the snapped
file system, the snapshot gradually fills with data copied from the snapped file
system.
The amount of disk space required for the snapshot depends on the rate of change
of the snapped 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.
Warning: If a snapshot file system runs out of space for changed data blocks, it is
disabled and all further attempts to access it fails. This does not affect the snapped
file system.
81Online backup using file system snapshots
How a snapshot file system works