[转贴]FreeBSD UFS2 Snapshots

eliumao

看来我要等到5.3出来的时候试试了


FreeBSD UFS2 Snapshots
Management Environment
How to Deliver FreeBSD UFS2 Snapshots to End-Users
Ralf S. Engelschall <rse@FreeBSD.org>;
Written: 29-Aug-2004, Last Modified: 03-Sep-2004
Problem

The FreeBSD 5 UFS2 filesystem provides the possibility to create snapshots of live filesystems. It is already best known (and can be easily used) for allowing fsck( to run in the background (see rc.conf variable background_fsck) and for allowing to create consistent filesystem dumps (see dump( option -L).

Additionally, with the help of md(4) and mdconfig( you can mount snapshots as read-only filesystems, too. For illustration purposes, look at this sample session where a snapshot of /var is created:
# echo "before" >;/var/tmp/foo.txt # ls -l /var/tmp/foo.txt -rw-r--r-- 1 root wheel 7 Sep 3 16:19 /var/tmp/foo.txt # test -d /var/.snap || mkdir /var/.snap # mount -u -o snapshot /var/.snap/test /var # ls -l /var/.snap/test -r-------- 1 root operator 2147483784 Sep 3 16:19 /var/.snap/test # echo "after" >;/var/tmp/foo.txt # ls -l /var/tmp/foo.txt -rw-r--r-- 1 root wheel 6 Sep 3 16:20 /var/tmp/foo.txt # mdconfig -a -t vnode -f /var/.snap/test -u 1 # mount -o ro /dev/md1 /mnt # ls -l /mnt/tmp/foo.txt /var/tmp/foo.txt -rw-r--r-- 1 root wheel 7 Sep 3 16:19 /mnt/tmp/foo.txt -rw-r--r-- 1 root wheel 6 Sep 3 16:20 /var/tmp/foo.txt # cat /mnt/tmp/foo.txt /var/tmp/foo.txt before after # umount /mnt # mdconfig -d -u 1 # rm -f /var/.snap/test

As you can see, one can easily create a snapshot and use it to recover older states of files. Unfortunately, this is all which currently is provided by FreeBSD out-of-the-box. There is neither an easy way provided to regularily create new and expire old snapshots nor an easy or even unpriviledged way to access the snapshot data. Additionally, although the involved steps and commands are not too complicated, they are not convenient enough for working with them daily.

Keep in mind that regular filesystem backup snapshots are a great solution for easily recovering old file states within a small time frame. But only if the interface for the user follows the KISS principle. And at least this is IMHO not the case under our FreeBSD until now.

On the other hand, at work my home directory is placed on an NFS mounted WAFL® filesystem provided by an ONTAP® based Filer from Network Appliance. For instance, after you have configured scheduled snapshots with the ONTAP® command "snap sched 0 0 4", you can easily at any time recover old states (as a regular user) in hourly steps for the last 4 hours:
$ echo "before" >;foo.txt $ sleep 3601 $ echo "middle" >;foo.txt $ sleep 3601 $ echo "after" >;foo.txt $ ls -la | fgrep .snapshot $ cat .snapshot/hourly.1/foo.txt .snapshot/hourly.0/foo.txt foo.txt before middle after
Solution
A similar service should be also provided to our users under FreeBSD, of course. We achieve this by implementing three solutions:

   1. Snapshot Management Frontend:
      The involved commands should be wrapped by a frontend utility snapshot( which provides a more simple and convenient way to create, expire, mount and unmount snaphots.
      # snapshot list /tmp # snapshot make -g3 /tmp:test # snapshot list /tmp /tmp test.0 # snapshot make -g3 /tmp:test # snapshot make -g3 /tmp:test # snapshot list /tmp /tmp test.0 /tmp test.1 /tmp test.2 # snapshot make -g3 /tmp:test # snapshot make -g3 /tmp:test # snapshot list /tmp /tmp test.0 /tmp test.1 /tmp test.2 # snapshot mount /tmp:test.0 /mnt # snapshot umount /mnt # snapshot make -g0 /tmp:test # snapshot list /tmp

   2. Periodic and Flexible Backup Snapshot Creation
      There has to be a flexible and easy way to configure periodically created backup snapshots. For instance, to have available two "daily" snapshots for / and /usr, two daily and four hourly (created every hour) for /var and two weekly, seven daily and eight hourly (created at times 08:00, 12:00, 16:00 and 20:00) snapshots for /home/ all which is required should be (similar to the syntax of the ONTAP "snap sched" command):
      $ grep ^snapshot_ /etc/periodic.conf snapshot_enable="YES" snapshot_schedule="/,/usr:0:2:0 /var:0:2:4 /home:2:7:8@8,12,16,20"

   3. Easy Access to Backup Snapshot Data
      There has to be an easy way to access the data of an arbitrary backup snapshot as a regular user. Mostly similar to the .snapshot sub-directory feature on WAFL (we use a top-level /snap directly here because this is what can be provided by amd( easily and without any performance penalties and stability issues):
      $ df | egrep '(/snap|/.am)'; ls -l /snap pid1562@en2:/snap 0 0 0 100% /snap $ cd /home/rse $ echo "before" >;foo.txt $ sleep 3601 $ echo "middle" >;foo.txt $ sleep 3601 $ echo "after" >;foo.txt $ cat /snap/home:hourly.1/rse/foo.txt /snap/home:hourly.0/rse/foo.txt foo.txt before middle after $ df | egrep '(/snap|/.am)'; ls -l /snap pid1562@en2:/snap 0 0 0 100% /snap /dev/md1 2026030 2464 1861484 0% /.am/en2/snap/var:hourly.0 lrwxrwxrwx 1 root wheel 26 Sep 3 16:38 var:hourly.0 ->; /.am/en2/snap/var:hourly.0 $ sleep 3600 $ df | egrep '(/snap|/.am)'; ls -l /snap pid1562@en2:/snap 0 0 0 100% /snap
      As the surrounding df(1) and ls(1) calls show, amd( on demand mounts the snapshot files and after about 5 minutes of no access it automatically unmounts them again.

With those three solutions, FreeBSD UFS2 snapshots are available to our unprivileged users as a short-time backup solution allowing them to easily recover their older file states.
Implementation
The implementation is straight-forward and based on mount(, mdconfig(, amd( and cron(:

   1. Snapshot Management Frontend:
      First, I've implemented a little utility snapshot(8) (implementation peek: /usr/sbin/snapshot and /usr/share/man/man8/snapshot.8) which provides the convenience frontend for creating, expiring, mounting and unmounting snapshots.

   2. Periodic and Flexible Backup Snapshot Creation
      Second, I've implemented a periodic scheduler periodic-snapshot(8) (implementation peek: periodic-snapshot and /usr/share/man/man8/periodic-snapshot.8) which is called from /etc/crontab (periodic(8) cannot be used here because we need hourly runs and at exact times to not confuse users) and runs the "snapshot make" commands according to the configuration provided in /etc/periodic.conf.

   3. Easy Access to Backup Snapshot Data
      Third, I've setup a amd(8) map /etc/amd.map.snap in /etc/rc.conf which performs the "snapshot mount" and "snapshot umount" commands when a user accesses directories under /snap.

In order to easily integrate this implementation into a FreeBSD 5-STABLE installation, DOWNLOAD the latest tarball HERE and install it as following:
# tar zxf freebsd-snapshot-*.tar.gz # cd freebsd-snapshot-* # make install
(If you later want to remove it again, just run "make uninstall"
References
See the following URLs for further details:

    * mount(8)
    * mksnap_ffs(8)
    * mdconfig(8)
    * amd(8)
    * /usr/src/sys/ufs/ffs/README.snapshot
    * http://www.mckusick.com/softdep/