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snapshot
This option allows a snapshot of the specified file sys-
tem to be taken. The -u flag is required with this
option. Note that snapshot files must be created in the
file system that is being snapshotted. You may create up
to 20 snapshots per file system. Active snapshots are
recorded in the superblock, so they persist across
unmount and remount operations and across system reboots.
When you are done with a snapshot, it can be removed with
the rm(1) command. Snapshots may be removed in any
order, however you may not get back all the space con-
tained in the snapshot as another snapshot may claim some
of the blocks that it is releasing. Note that the schg
flag is set on snapshots to ensure that not even the root
user can write to them. The unlink command makes an
exception for snapshot files in that it allows them to be
removed even though they have the schg flag set, so it is
not necessary to clear the schg flag before removing a
snapshot file.
Once you have taken a snapshot, there are three interest-
ing things that you can do with it:
1. Run fsck(8) on the snapshot file. Assuming that the
file system was clean when it was mounted, you
should always get a clean (and unchanging) result
from running fsck on the snapshot. This is essen-
tially what the background fsck process does.
2. Run dump(8) on the snapshot. You will get a dump
that is consistent with the file system as of the
timestamp of the snapshot.
3. Mount the snapshot as a frozen image of the file
system. To mount the snapshot /var/snapshot/snap1:
mdconfig -a -t vnode -f /var/snapshot/snap1 -u 4
mount -r /dev/md4 /mnt
You can now cruise around your frozen /var file sys-
tem at /mnt. Everything will be in the same state
that it was at the time the snapshot was taken. The
one exception is that any earlier snapshots will
appear as zero length files. When you are done with
the mounted snapshot:
umount /mnt
mdconfig -d -u 4
Further details can be found in the file at
/usr/src/sys/ufs/ffs/README.snapshot.
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