Re: BTRFS messes up snapshot LV with origin

[Goffredo Baroncelli <kreijack@inwind.it>]

On 11/21/2014 05:28 AM, Zygo Blaxell wrote:
> e.g. if an ext4 filesystem explodes, I can:
> 
> 	1.  make a LVM snapshot of the broken filesystem
> 
> 	2.  run e2fsck on the snapshot
> 
> 	3.  mount and repair the snapshot, e.g. rsync any missing files
> 	from backups, salvage anything that survived
> 
> 	4.  LVM merge the snapshot to its origin volume
> 
> 	5.  umount the origin volume and mount the merged volume
> 	(or just reboot)
> 
> ...and I can do all of this on a running system, in-place, with only a
> few minutes of downtime in the must-reboot case.
> 
> None of the above works with btrfs at all.  Multi-device btrfs fails
> at 2, 

You can't compare ext4 with btrfs, if you are talking about a multi-device 
filesystem: ext4 haven't this capability. 
Try to make a md-raid over a snapshotted logical volume(s); I never tried
that, but I suppose that there will be the same problems...

> and mounting the filesystem fails at 3.  
Are you sure ?

ghigo@venice:/tmp$ # create a btrfs filesystem in a logical volume
ghigo@venice:/tmp$ sudo truncate -s +10G disk.img
ghigo@venice:/tmp$ sudo losetup -f disk.img 
ghigo@venice:/tmp$ sudo pvcreate /dev/loop0 
ghigo@venice:/tmp$ sudo vgcreate vgtest /dev/loop0 
ghigo@venice:/tmp$ sudo lvcreate -n lvone -L 3G vgtest
ghigo@venice:/tmp$ sudo mkfs.btrfs /dev/vgtest/lvone 
ghigo@venice:/tmp$ mkdir t

ghigo@venice:/tmp$ # create a file inside a btrfs fs
ghigo@venice:/tmp$ sudo mount /dev/vgtest/lvone t/
ghigo@venice:/tmp$ sudo dd if=/dev/zero of=t/disk-orig bs=1M count=1
ghigo@venice:/tmp$ sudo umount t

ghigo@venice:/tmp$ # make a lvm snapshot and add a 2nd file
ghigo@venice:/tmp$ sudo lvcreate -s -n lvone_snap -L 3G vgtest/lvone
ghigo@venice:/tmp$ sudo mount /dev/vgtest/lvone_snap t/
ghigo@venice:/tmp$ sudo dd if=/dev/zero of=t/disk-snap bs=1M count=1
ghigo@venice:/tmp$ sudo umount t

ghigo@venice:/tmp$ # mount the first one lv, and check the file
ghigo@venice:/tmp$ sudo mount /dev/vgtest/lvone t/
ghigo@venice:/tmp$ ls -l t
total 1024
-rw-r--r-- 1 root root 1048576 Nov 22 18:11 disk-orig
ghigo@venice:/tmp$ sudo umount t

ghigo@venice:/tmp$ # mount the first one lv, and check the files
ghigo@venice:/tmp$ sudo mount /dev/vgtest/lvone_snap t/
ghigo@venice:/tmp$ ls -l t
total 2048
-rw-r--r-- 1 root root 1048576 Nov 22 18:11 disk-orig
-rw-r--r-- 1 root root 1048576 Nov 22 18:12 disk-snap

On the basis of the example above, in case you want to mount a 
"single-disk", BTRFS seems me to work properly. You have to pay
attention only to not mount the two filesystem at the same time.

BR
G.Baroncelli


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