Contributed by Tom Rhodes.
FreeBSD 5.0 offers a new feature in conjunction with Soft Updates: File system snapshots.
Snapshots allow a user to create images of specified file systems, and treat them as a
file. Snapshot files must be created in the file system that the action is performed on,
and a user may create no more than 20 snapshots per file system. Active snapshots are
recorded in the superblock so they are persistent across unmount and remount operations
along with system reboots. When a snapshot is no longer required, it can be removed with
the standard rm(1) command.
Snapshots may be removed in any order, however all the used space may not be acquired
because another snapshot will possibly claim some of the released blocks.
During initial creation, the schg flag (see the chflags(1) manual
page) is set to ensure that even root cannot write to the
snapshot. The unlink(1) command
makes an exception for snapshot files since it allows them to be removed with the schg flag set, so it is not necessary to clear the schg flag before removing a snapshot file.
Snapshots are created with the mount(8) command. To
place a snapshot of /var in the file /var/snapshot/snap use the following command:
# mount -u -o snapshot /var/snapshot/snap /var
Alternatively, you can use mksnap_ffs(8) to
create a snapshot:
# mksnap_ffs /var /var/snapshot/snap
Once a snapshot has been created, it has several uses:
-
Some administrators will use a snapshot file for backup purposes, because the snapshot
can be transfered to CDs or tape.
-
File integrity, fsck(8) may be ran on
the snapshot. Assuming that the file system was clean when it was mounted, you should
always get a clean (and unchanging) result. This is essentially what the background fsck(8) process
does.
-
Run the dump(8) utility on the
snapshot. A dump will be returned that is consistent with the file system and the
timestamp of the snapshot. dump(8) can also take
a snapshot, create a dump image and then remove the snapshot in one command using the
-L flag.
-
mount(8) the snapshot
as a frozen image of the file system. To mount(8) the snapshot
/var/snapshot/snap run:
# mdconfig -a -t vnode -f /var/snapshot/snap -u 4
# mount -r /dev/md4 /mnt
You can now walk the hierarchy of your frozen /var file
system mounted at /mnt. Everything will be in the same state it
was during the snapshot creation time. The only exception is that any earlier snapshots
will appear as zero length files. When the use of a snapshot has delimited, it can be
unmounted with:
# umount /mnt
# mdconfig -d -u 4
For more information about softupdates and file system
snapshots, including technical papers, you can visit Marshall Kirk McKusick's website at
http://www.mckusick.com.
