Veritas Storage Foundation 5.1 SP1 Advanced Features Administrator"s Guide (5900-1503, April 2011)

unmounted until all of its snapshots are unmounted. Although 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.
Note: A snapshot file system ceases to exist when unmounted. If mounted again,
it is actually a fresh snapshot of the snapped file system. A snapshot file system
must be unmounted before its dependent snapped file system can be unmounted.
Neither the fuser command nor the mount command will indicate that a snapped
file system cannot be unmounted because a snapshot of it exists.
On cluster file systems, snapshots can be created on any node in the cluster, and
backup operations can be performed from that node. The snapshot of a cluster
file system is accessible only on the node where it is created, that is, the snapshot
file system itself cannot be cluster mounted.
See the Veritas Storage Foundation Cluster File System Administrator's Guide.
How a snapshot file system works
A snapshot file system is created by mounting an empty disk slice as a snapshot
of a currently mounted file system. The bitmap, blockmap and super-block are
initialized and then the currently mounted file system is frozen. After the file
system to be snapped is frozen, the snapshot is enabled and mounted and the
snapped file system is thawed. The snapshot appears as an exact image of the
snapped file system at the time the snapshot was made.
Initially, the snapshot file system satisfies read requests by 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 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
Administering snapshot file systems
How a snapshot file system works
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