Hi,
Find instructions on how to recreate below -
I have a BTRFS raid 10 setup in Virtualbox (I'm getting to grips with
the Filesystem)
I have the raid mounted to /mnt like so -
[root@Xen ~]# btrfs filesystem show /mnt/
Label: none uuid: ad1d95ee-5cdc-420f-ad30-bd16158ad8cb
Total devices 4 FS bytes used 1.00GiB
devid 1 size 2.00GiB used 927.00MiB path /dev/sdb
devid 2 size 2.00GiB used 927.00MiB path /dev/sdc
devid 3 size 2.00GiB used 927.00MiB path /dev/sdd
devid 4 size 2.00GiB used 927.00MiB path /dev/sde
And -
[root@Xen ~]# btrfs filesystem usage /mnt/
Overall:
Device size: 8.00GiB
Device allocated: 3.62GiB
Device unallocated: 4.38GiB
Device missing: 0.00B
Used: 2.00GiB
Free (estimated): 2.69GiB (min: 2.69GiB)
Data ratio: 2.00
Metadata ratio: 2.00
Global reserve: 16.00MiB (used: 0.00B)
Data,RAID10: Size:1.50GiB, Used:1.00GiB
/dev/sdb 383.50MiB
/dev/sdc 383.50MiB
/dev/sdd 383.50MiB
/dev/sde 383.50MiB
Metadata,RAID10: Size:256.00MiB, Used:1.16MiB
/dev/sdb 64.00MiB
/dev/sdc 64.00MiB
/dev/sdd 64.00MiB
/dev/sde 64.00MiB
System,RAID10: Size:64.00MiB, Used:16.00KiB
/dev/sdb 16.00MiB
/dev/sdc 16.00MiB
/dev/sdd 16.00MiB
/dev/sde 16.00MiB
Unallocated:
/dev/sdb 1.55GiB
/dev/sdc 1.55GiB
/dev/sdd 1.55GiB
/dev/sde 1.55GiB
Right so everything looks good and I stuck some dummy files in there too
-
[root@Xen ~]# ls -lh /mnt/
total 1.1G
-rw-r--r-- 1 root root 1.0G May 30 2008 1GB.zip
-rw-r--r-- 1 root root 28 Mar 24 15:16 hello
-rw-r--r-- 1 root root 6 Mar 24 16:12 niglu
-rw-r--r-- 1 root root 4 Mar 24 15:32 test
The bug appears to happen when you try and test out it's ability to
handle a dead drive.
If you follow the instructions here:
https://btrfs.wiki.kernel.org/index.php/Using_Btrfs_with_Multiple_Devices#Replacing_failed_devices
It tells you do mount the drive with the 'degraded' option,, however
this just does not work, allow me to show -
1) I power off the VM and remove one of the drives (Simulating a drive
being pulled from a machine)
2) Power on the VM
3) Check DMESG - Everything looks good
4) Check how BTRFS is feeling -
Label: none uuid: ad1d95ee-5cdc-420f-ad30-bd16158ad8cb
Total devices 4 FS bytes used 1.00GiB
devid 1 size 2.00GiB used 1.31GiB path /dev/sdb
devid 2 size 2.00GiB used 1.31GiB path /dev/sdc
devid 3 size 2.00GiB used 1.31GiB path /dev/sdd
*** Some devices missing
So far so good, /dev/sde is missing and BTRFS has detected this.
5) Try and mount it as per the wiki so I can remove the bad drive and
replace it with a good one -
[root@Xen ~]# mount -o degraded /dev/sdb /mnt/
mount: wrong fs type, bad option, bad superblock on /dev/sdb,
missing codepage or helper program, or other error
In some cases useful info is found in syslog - try
dmesg | tail or so.
Ok, this is not good, I check DMESG -
[root@Xen ~]# dmesg | tail
[ 4.416445] e1000: enp0s3 NIC Link is Up 1000 Mbps Full Duplex, Flow
Control: RX
[ 4.416672] IPv6: ADDRCONF(NETDEV_CHANGE): enp0s3: link becomes ready
[ 4.631812] snd_intel8x0 0000:00:05.0: white list rate for 1028:0177
is 48000
[ 7.091047] floppy0: no floppy controllers found
[ 27.488345] BTRFS info (device sdb): allowing degraded mounts
[ 27.488348] BTRFS info (device sdb): disk space caching is enabled
[ 27.488349] BTRFS: has skinny extents
[ 27.489794] BTRFS warning (device sdb): devid 4 uuid
ebcd53d9-5956-41d9-b0ef-c59d08e5830f is missing
[ 27.491465] BTRFS: missing devices(1) exceeds the limit(0), writeable
mount is not allowed
[ 27.520231] BTRFS: open_ctree failed
So here lies the problem - BTRFS needs you to have all the devices
present in order to mount is as writeable, however if a drive dies
spectacularly (as they can do) You can't have that do that. And as a
result you cannot mount any of the remaining drives and fix the problem.
Now you ARE able to mount it read only but you can't issue the fix that
is recommend on the wiki, see here -
[root@Xen ~]# mount -o ro,degraded /dev/sdb /mnt/
[root@Xen ~]# btrfs device delete missing /mnt/
ERROR: error removing device 'missing': Read-only file system
So catch 22, you need all the drives otherwise it won't let you mount,
But what happens if a drive dies and the OS doesn't detect it? BTRFS
wont allow you to mount the raid volume to remove the bad disk!
I also tried it with read only -
[root@Xen ~]# mount -o ro,degraded /dev/sdb /mnt/
[root@Xen ~]# btrfs device delete missing /mnt/
ERROR: error removing device 'missing': Read-only file system
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