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Add a bit of information on FFS snapshots to the mount(8) manual page.

Browse Source 
PR:		30139
Submitted by:	Chris Knight <chris@aims.com.au?
MFC after:	3 days
This commit is contained in:
Tom Rhodes 2002-07-11 21:12:45 +00:00
parent 4f5d913371
commit 299f15612e
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=99817
1 changed files with 67 additions and 0 deletions
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sbin/mount/mount.8
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@ -180,6 +180,73 @@ mount the filesystem read-only (even the super-user may not write it).
All
.Tn I/O
to the filesystem should be done synchronously.
.It Cm snapshot
This option allows a snapshot of the specified filesystem to be taken.
The
.Fl u
flag is required with this option.
Note that snapshot files must be created in the filesystem that is being
snapshotted.
You may create up to 20 snapshots per filesystem.
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
.Xr rm
command.
Snapshots may be removed in any order, however you may not get back all the
space contained 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.
.Pp
Once you have taken a snapshot, there are three interesting things that you can
do with it:
.Pp
.Bl -enum -compact
.It
Run fsck on the snapshot file.
Assuming that the filesystem was clean when it was mounted, you should always
get a clean (and unchanging) result from running fsck on the snapshot.
This is essentially what the background fsck process does.
.Pp
.It
Run dump on the snapshot.
You will get a dump that is consistent with the filesystem as of the timestamp
of the snapshot.
Note that
.Xr dump
has not yet been changed to set the dumpdates file correctly, so do not use this
feature in production until that fix is made.
.Pp
.It
Mount the snapshot as a frozen image of the filesystem.
To mount the snapshot
.Pa /var/snapshot/snap1 :
.Bd -literal
mdconfig -a -t vnode -f /var/snapshot/snap1 -u 4
mount -r /dev/md4 /mnt
.Ed
.Pp
You can now cruise around your frozen
.Pa /var
filesystem at
.Pa /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:
.Bd -literal
umount /mnt
mdconfig -d -u 4
.Ed
.Pp
Further details can be found in the file at
.Pa /usr/src/sys/ufs/ffs/README.snapshot .
.El
.It Cm suiddir
A directory on the mounted filesystem will respond to the SUID bit
being set, by setting the owner of any new files to be the same