Hello,
I was trying to create a multi-device Btrfs filesystem using two
loopback devices, by referring to the following page:
https://btrfs.wiki.kernel.org/index.php/Using_Btrfs_with_Multiple_Devices
However, I met a strange thing while mounting the first loop device
without any extra mount options,
1. Create and format two images, the 1st in 400Mbytes, and 2nd in
286Mbytes.
root@pibroch:/btrfs-progs# ls -lh /usr/src/linux-3.0/img*
-rw-r--r-- 1 jeff jeff 400M 2011-09-07 12:00 /usr/src/linux-3.0/img0
-rw-r--r-- 1 jeff jeff 286M 2011-09-07 12:00 /usr/src/linux-3.0/img1
root@pibroch:/btrfs-progs# mkfs.btrfs /usr/src/linux-3.0/img0
/usr/src/linux-3.0/img1
WARNING! - Btrfs v0.19-35-g1b444cd IS EXPERIMENTAL
WARNING! - see http://btrfs.wiki.kernel.org before using
failed to read /dev/sr0
failed to read /dev/sr0
adding device /usr/src/linux-3.0/img1 id 2
fs created label (null) on /usr/src/linux-3.0/img0
nodesize 4096 leafsize 4096 sectorsize 4096 size 685.50MB
Btrfs v0.19-35-g1b444cd
2. Setup the loopback manually:
root@pibroch:/btrfs-progs# losetup /dev/loop0 /usr/src/linux-3.0/img0
root@pibroch:/btrfs-progs# losetup /dev/loop1 /usr/src/linux-3.0/img1
root@pibroch:/btrfs-progs# losetup -a
/dev/loop0: [0802]:1736614 (/usr/src/linux-3.0/img0)
/dev/loop1: [0802]:1736726 (/usr/src/linux-3.0/img1)
3. Try to mount loop0, it will failed as below:
root@pibroch:/btrfs-progs# mount /dev/loop0 /mnt
mount: wrong fs type, bad option, bad superblock on /dev/loop0,
missing codepage or helper program, or other error
In some cases useful info is found in syslog - try
dmesg | tail or so
4. Try to mount loop1, it works:
root@pibroch:/btrfs-progs# mount /dev/loop1 /mnt
root@pibroch:/btrfs-progs# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/sda2 46G 23G 21G 52% /
none 896M 700K 895M 1% /dev
none 957M 80K 957M 1% /dev/shm
none 957M 104K 957M 1% /var/run
none 957M 0 957M 0% /var/lock
/dev/loop1 686M 56K 493M 1% /mnt
5. umount loop1, and try to mount loop0 again, it works at this time:
root@pibroch:/btrfs-progs# umount /mnt
root@pibroch:/btrfs-progs# mount /dev/loop0 /mnt
root@pibroch:/btrfs-progs# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/sda2 46G 23G 21G 52% /
none 896M 700K 895M 1% /dev
none 957M 80K 957M 1% /dev/shm
none 957M 104K 957M 1% /var/run
none 957M 0 957M 0% /var/lock
/dev/loop0 686M 56K 493M 1% /mnt
According to my debugging result, for the 1st time, mount loop0 failed
at around volumes.c:3468:
map->stripes[i].dev = btrfs_find_device(root, devid, uuid,
NULL);
if (!map->stripes[i].dev && !btrfs_test_opt(root,
DEGRADED)) {
kfree(map);
free_extent_map(em);
return -EIO;
So I repeat the above steps again, and try to mount loop0 with "-o
degraded" option, it works as expected:
root@pibroch:/btrfs-progs# mount -o degraded /dev/loop0 /mnt
root@pibroch:/btrfs-progs# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/sda2 46G 23G 21G 52% /
none 896M 700K 895M 1% /dev
none 957M 140K 957M 1% /dev/shm
none 957M 104K 957M 1% /var/run
none 957M 0 957M 0% /var/lock
/dev/loop0 686M 56K 136M 1% /mnt
kernel version 3.1.0-rc2+.
Is it a bug? or Am I missing something?
Thanks,
-Jeff
--
To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
