OS Upgrade and Rollback Using BTRFS

I recently decided to try out the snapshotting capabilities of the relatively new BTRFS filesystem. I have been using the snapshot and rollback capability of LVM (using lvconvert --merge) for a while now so I figured I would check out BTRFS to see how it stacks up.

To get up to speed on how to use BTRFS I found the BTRFS Fun web page a good reference. I converted an existing Fedora 17 virtual machine to use BTRFS for the filesystems (I may cover how I did this in a later post). The disk inside the virtual machine contains three partitions as is shown below:

[root@guest1 ~]# lsblk NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT sr0 11:0 1 4.2G 0 rom vda 252:0 0 20G 0 disk ├─vda1 252:1 0 500M 0 part /boot ├─vda2 252:2 0 4G 0 part [SWAP] └─vda3 252:3 0 15.6G 0 part / [root@guest1 ~]#

As you can see from the output, vda1 is for /boot while vda3 is for /; the root partition. After converting from ext4, both vda1 and vda3 have BTRFS volumes (filesystems) on them. Now I am free to snapshot them as I please.

Taking The Snapshots

In BTRFS snapshots are actually subvolumes (see here for an explanation of subvolumes). This means when I create a snapshot of /boot and / it is actually creating a subvolume.

To create the snapshots you use the btrfs subvolume snapshot command and you must specify where in the filesystem you want the snapshot to reside. To help myself not get confused I created a /boot/.snapshots directory and a /.snapshots directory which is where I will create the snapshots. The creation of the snapshots is shown below:

[root@guest1 ~]# mkdir /boot/.snapshots [root@guest1 ~]# mkdir /.snapshots [root@guest1 ~]# [root@guest1 ~]# btrfs subvolume snapshot /boot /boot/.snapshots/snap1 Create a snapshot of '/boot' in '/boot/.snapshots/snap1' [root@guest1 ~]# [root@guest1 ~]# btrfs subvolume snapshot / /.snapshots/snap1 Create a snapshot of '/' in '/.snapshots/snap1' [root@guest1 ~]# [root@guest1 ~]# [root@guest1 ~]# btrfs subvolume list /boot ID 256 top level 5 path .snapshots/snap1 [root@guest1 ~]# [root@guest1 ~]# btrfs subvolume list / ID 270 top level 5 path .snapshots/snap1 [root@guest1 ~]#

If you notice in the output of the btrfs subvolume list commands above, the subvolumes (snapshots) that were created have a unique ID associated with them. For each BTRFS filesystem there is also an implicit "root subvolume" with an ID of 0.

These IDs can be used to set which subvolume is the one that is used when the BTRFS filesystem is mounted. In order to be able to easily identify if I had mounted a snapshot or the root subvolume, I placed a single file snap1_sub into the root directory of each snapshot.

[root@guest1 ~]# touch /boot/.snapshots/snap1/snap1_sub [root@guest1 ~]# [root@guest1 ~]# touch /.snapshots/snap1/snap1_sub

Upgrade From F17 to F18 Beta

At this point the snapshots were all set up and I could go ahead with upgrading from Fedora 17 to the Fedora 18 beta. I followed the steps from the Fedora Web Site on how to do this. After rebooting I indeed had upgraded to Fedora 18:
[root@guest1 ~]# cat /etc/redhat-release Fedora release 18 (Spherical Cow) [root@guest1 ~]#

However, as usual the upgrade didn't yield a perfectly working system. As far as the command line was concerned everything seemed to be fine but my desktop session was a no go and the logs indicated there was something awry with the X configuration.

No need to fear! The snapshots are there!

Reverting The Snapshots

What we want to do now is revert the changes and go back to Fedora 17. How do we do that?

One way is to make the snapshot subvolume be the default subvolume that is used when the filesystem is mounted. I set both /boot and / to default to their snapshots by using the btrfs subvolume set-default command:

[root@guest1 ~]# btrfs subvolume list /boot/ ID 256 gen 29 top level 5 path .snapshots/snap1 [root@guest1 ~]# [root@guest1 ~]# btrfs subvolume set-default 256 /boot/ [root@guest1 ~]# [root@guest1 ~]# [root@guest1 ~]# [root@guest1 ~]# btrfs subvolume list / ID 270 gen 139 top level 5 path .snapshots/snap1 [root@guest1 ~]# [root@guest1 ~]# btrfs subvolume set-default 270 / [root@guest1 ~]# [root@guest1 ~]# reboot

After a quick reboot I was back up and running. I wanted to verify I was in the snapshot so I checked to see if the files I had created were in the root of the filesystems:

[root@guest1 ~]# ls -l /boot/snap1_sub -rw-r--r-- 1 root root 0 Jan 4 23:20 /boot/snap1_sub [root@guest1 ~]# [root@guest1 ~]# ls -l /snap1_sub -rw-r--r-- 1 root root 0 Jan 4 23:20 /snap1_sub [root@guest1 ~]# [root@guest1 ~]# cat /etc/redhat-release Fedora release 17 (Beefy Miracle)

The next step was to restore the root subvolume (ID=0) of each filesystem. I started with /boot (from vda1). To do this you mount the root subvolume and then use rsync to restore any files that were changed.

NOTE: In the rsync command I was careful to exclude the file that I created to let me know I was in the snapshot as well as the directory that is used for the snapshots.

[root@guest1 ~]# mount -o subvolid=0 /dev/vda1 /mnt [root@guest1 ~]# [root@guest1 ~]# time rsync --delete -avHAX --exclude=snap1_sub --exclude=/.snapshots /boot/ /mnt/ sending incremental file list ./ deleting vmlinuz-3.6.10-4.fc18.x86_64 deleting initrd-plymouth.img deleting initramfs-3.6.10-4.fc18.x86_64.img deleting config-3.6.10-4.fc18.x86_64 deleting System.map-3.6.10-4.fc18.x86_64 deleting .vmlinuz-3.6.10-4.fc18.x86_64.hmac grub/ grub/splash.xpm.gz grub2/ grub2/grub.cfg grub2/themes/ grub2/themes/system/ deleting grub2/themes/system/unicode.pf2 deleting grub2/themes/system/fireworks.png deleting grub2/themes/system/DejaVuSans-Bold-14.pf2 deleting grub2/themes/system/DejaVuSans-12.pf2 deleting grub2/themes/system/DejaVuSans-10.pf2 grub2/themes/system/background.png grub2/themes/system/dejavu.pf2 grub2/themes/system/theme.txt sent 4216333 bytes received 234 bytes 8433134.00 bytes/sec total size is 36454169 speedup is 8.65 real 0m0.135s user 0m0.023s sys 0m0.018s [root@guest1 ~]#

From the output we can see that upgrading caused about 34M (36454169 bytes) of changes to the filesystem and it took less than a second to revert the changes.

Now that the /boot filesystem has been restored we need to set the default subvolume to mount again. We need to reset it back to automatically mount the root subvolume.

NOTE: When setting the default subvolume you have to be careful and make sure to specify the location where the root subvolume is mounted (in this case /mnt). There is a small bit of text that attempts to explain why this is necessary on the BTRFS Fun web page.
[root@guest1 ~]# btrfs subvolume set-default 0 /mnt [root@guest1 ~]# umount /mnt

Now on to the / filesystem. For this one I needed to tweak the rsync command to make sure that it didn't attempt to sync anything outside of the filesystem; we don't care to sync fake filesystems like /dev /proc /sys etc.. Also we wanted to exclude /mnt since that is where the root subvolume is mounted.

[root@guest1 ~]# mount -o subvolid=0 /dev/vda3 /mnt [root@guest1 ~]# [root@guest1 ~]# time rsync --one-file-system --delete -avHAX --exclude=/snap1_sub --exclude=/.snapshots --exclude=/mnt / /mnt sending incremental file list ... ... sent 2822450331 bytes received 3425590 bytes 5429156.43 bytes/sec total size is 3109809657 speedup is 1.10 real 8m40.348s user 0m19.899s sys 0m36.568s [root@guest1 ~]# [root@guest1 ~]# [root@guest1 ~]# btrfs subvolume set-default 0 /mnt [root@guest1 ~]# umount /mnt [root@guest1 ~]# [root@guest1 ~]# reboot

And we are done.. It took less than 10 minutes to revert ~3G of changes that were made. After the reboot my Fedora 17 instance was fully restored and we were in the root subvolumes as can be seen below from the absence of the snap1_sub files that I had created.

[root@guest1 ~]# cat /etc/redhat-release Fedora release 17 (Beefy Miracle) [root@guest1 ~]# [root@guest1 ~]# ls -l /boot/snap1_sub ls: cannot access /boot/snap1_sub: No such file or directory [root@guest1 ~]# [root@guest1 ~]# ls -l /snap1_sub ls: cannot access /snap1_sub: No such file or directory [root@guest1 ~]# [root@guest1 ~]#

The final step was to delete the snapshots as they are no longer needed.

[root@guest1 ~]# btrfs subvolume delete /boot/.snapshots/snap1 Delete subvolume '/boot/.snapshots/snap1' [root@guest1 ~]# [root@guest1 ~]# btrfs subvolume delete /.snapshots/snap1 Delete subvolume '/.snapshots/snap1'


Overall I believe the snapshotting feature of BTRFS is quite useful. I like the fact that you don't have to carve out new storage for the snapshots like in LVM, but at the same time I don't like how it doesn't seem to be very easy to detect how much space the snapshots are using up. I also don't like the fact that reverting the snapshots are a bit manual and require an external tool like rsync. In LVM it is handled all in one command: lvconvert --merge. That being said, just from experience it does seem that reverting the snapshots seems to be a bit faster with BTRFS than with LVM snapshots. I would have to run another test to be sure of this.
Happy New Year Everyone!