BTRFS Subvolume of Home?

[Timberbrook]

Hey guys, I've been trying out BTRFS the last couple days, just to kind of see what to expect and what I can do with it once its finalized and we have a working fsck. I do have a couple question that neither the Arch Wiki, The Fedora WIki, Ubuntu wiki or the kernel.org wiki could answer. So here goes;


When I installed fedora (F16 Beta) I made root be an ext4 partition, with /home as BTRFS. (Side question; seriously, why cant Anaconda install as non-ext4 root? No other installer mandates the root partition be the same type as the liveCD). Ubuntu wiki states that any BTRFS partition has a default subvolume called "default," I cant find it for /home.

/etc/fstab:

#
# /etc/fstab
# Created by anaconda on Thu Nov 3 15:24:14 2011
#
# Accessible filesystems, by reference, are maintained under '/dev/disk'
# See man pages fstab(5), findfs(8), mount(8) and/or blkid(8) for more info
#
UUID=e5021a48-c961-4e73-98a5-beee06355f46 / ext4 defaults 1 1
UUID=97ac420f-2818-4833-b7b6-4ac497716e3b /home btrfs defaults,autodefrag,compress 1 2
UUID=0d275462-1ad3-46af-a1b8-637ca5efeca6 swap swap defaults 0 0


btrfs --help says. "btrfs subvolume list <path> : List the snapshot/subvolume of a filesystem." I'd expect it to spit out "default" at some point.

Couple outputs

[eric@eric-laptop ~]$ sudo btrfs subvolume list /
ERROR: '/' is not a subvolume
[eric@eric-laptop ~]$ sudo btrfs subvolume list /home
[eric@eric-laptop ~]$

What i wanted was an easy; reliable way to keep a backup of my home using btrfs instead of just rsync. Unfortunately, in order to use snapshot, it needs to be on, or be a, subvolume.

proof:

[eric@eric-laptop ~]$ sudo btrfs subvolume snapshot /home/eric /home/eric/test
ERROR: '/home/eric' is not a subvolume
[eric@eric-laptop ~]$

So; I rebooted into single user, mv'ed /home/eric to /home/eric-btrfs, made a subvolume in /home/ named 'eric' to take the place of my home, and cp'ed all of my files from eric-btrfs to eric, a quick chown, and chmod to get the file permissions right.

Rebooted, tried to login... error messages, lots of error messages. "Cant switch out from /" "/home/eric/.kde/share/config/knotifyrc is not writable" despite having read and write permission, and ownership.

Can anyone figure out what the heck is going on? I know BTRFS is still experimental, but I thought the layout and implementation was finalized. OR; what do I have to do to make Btrfs work as a /home as it should? (see ZFS / LVM )

[Timberbrook]

Anyone got ideas? I really want to have this worked out before F16 officially launches

[chrismurphy]

Originally Posted by Timberbrook

(Side question; seriously, why cant Anaconda install as non-ext4 root? No other installer mandates the root partition be the same type as the liveCD).

Pretty sure Fedora installer uses dd to write a root image to your root directory and that image is ext4. That's why the installation is super fast compared to the DVD installer. Then it simply resizes that lv postinstall. You can convert that ext4 to btrfs, I'm pretty sure the commands are included by default.

[Timberbrook]

Thanks for the reply Chris, atleast that explains Anaconda. But what does converting ext4 to btrfs get me? Its still not listing the 'default' subvolume thats supposed to exist so I can't do snapshots. Like this seems to be a pretty massive hole in the entire idea behind BTRFS' backups, if it doesnt create a default subvolume considering you need a subvolume to use snapshots.

[chrismurphy]

My understanding is that if you convert ext4 to btrfs, the original ext4 is a snapshot and everything else is btrfs. You might need to join one of the btrfs mailing lists and post some inquiries there. The dev list doesn't get insane amount of traffic so if you've read all the btrfs wiki stuff and have done at least some basic searches of the btrfs mailing list for keywords related to your question, I think you'd find some people who'd answer concise questions on that list.

[stevea]

Originally Posted by Timberbrook

Hey guys, I've been trying out BTRFS the last couple days, just to kind of see what to expect and what I can do with it once its finalized and we have a working fsck. I do have a couple question that neither the Arch Wiki, The Fedora WIki, Ubuntu wiki or the kernel.org wiki could answer. So here goes;
...
Ubuntu wiki states that any BTRFS partition has a default subvolume called "default," I cant find it for /home.
...

btrfs --help says. "btrfs subvolume list <path> : List the snapshot/subvolume of a filesystem." I'd expect it to spit out "default" at some point.

Couple outputs

[eric@eric-laptop ~]$ sudo btrfs subvolume list /
ERROR: '/' is not a subvolume
[eric@eric-laptop ~]$ sudo btrfs subvolume list /home
[eric@eric-laptop ~]$

I *think* the Ubuntu doc was implying there is always the root tree on the btrfs volume in addition to any subvolumes.

[root@hypoxylon ~]# mount -t btrfs /dev/sda6 /media/mp
[root@hypoxylon ~]# btrfs sub list  /media/mp
ID 256 top level 5 path newhome/stevea/SV-stevea
ID 257 top level 5 path newhome/testuser/SV-testuser
[root@hypoxylon ~]# mount -t btrfs -o subvolid=256 /dev/sda6 /media/mp1
[root@hypoxylon ~]# ls -l /media/mp1
total 0
-rw-r--r-- 1 root root 0 Nov  5 00:58 sv-s-bar
-rw-r--r-- 1 root root 0 Nov  5 00:58 sv-s-bletch
-rw-r--r-- 1 root root 0 Nov  5 00:58 sv-s-foo
[root@hypoxylon ~]# ls -l /media/mp/newhome/stevea/SV-stevea/
total 0
-rw-r--r-- 1 root root 0 Nov  5 00:58 sv-s-bar
-rw-r--r-- 1 root root 0 Nov  5 00:58 sv-s-bletch
-rw-r--r-- 1 root root 0 Nov  5 00:58 sv-s-foo

For whatever reason I can get "mount -o subvolid=..." to work but not mount -o subvol=...".

What i wanted was an easy; reliable way to keep a backup of my home using btrfs instead of just rsync. Unfortunately, in order to use snapshot, it needs to be on, or be a, subvolume.

I guess that makes sense, so the approach would be to snapshot the mounted subvolume at /home or whatever - right ? What's the difficulty ?
UUID=97ac420f-2818-4833-b7b6-4ac497716e3b /home btrfs subvolid=257,autodefrag,compress 1 2

So; I rebooted into single user, mv'ed /home/eric to /home/eric-btrfs, made a subvolume in /home/ named 'eric' to take the place of my home, and cp'ed all of my files from eric-btrfs to eric, a quick chown, and chmod to get the file permissions right.

Bad approach - you want to use "cp -prZ from to" to keep the permissions, ownership and SEL context intact. Fixing them after the fact is error prone and difficult.
[/QUOTE]

I'm a btrfs noob too. Hope that is of some help.