BTRFS root no space left on device, auto remount read-only; can't balance, can't add device

1

My BTRFs root filled up during system update and has been automatically remounted as read-only. I have plenty of files I can erase, but it's read only. I tried:

  1. sudo mount -o remount,rw / → mount point not mounted or bad option

  2. create a /dev/loop0 with a temporary 10G file and try to add it to the BTRFS device: sudo btrfs device add /dev/loop0 / → error adding device /dev/loop0: Read-only file system

  3. Rebalance: sudo btrfs balance start -dusage=0 / (also tried -musage=0) → error during balancing '/': Read-only file system

So basically everything I tried fails because the filesystem is read only. I can't remount it as rw for some reason. I'm afraid to reboot the system as it will probably not boot. Is there no hope for someone with a btrfs full root fs?

Piotr Kempa

Posted 2019-03-29T14:42:07.257

Reputation: 239

Can you mount it elsewhere? My / is mounted from /@ btrfs subvolume, but the whole btrfs root (/) is mounted as /mnt/ssd, so /mnt/ssd/@ duplicates (parts of) my / in some sense (please see this if it's not clear enough). I'm not sure if one of them going read-only would cause the other become read-only as well. The point is it's possible to mount the same filesystem twice. Have you tried?

– Kamil Maciorowski – 2019-03-29T15:01:04.607

Yes, I did try, to an empty directory /mnt2 (/mnt was used), it's not possible: sudo mount /dev/sda5 /mnt2 mount: /mnt2: wrong fs type, bad option, bad superblock on /dev/sda5, missing codepage or helper program, or other error.

Edit: I'm not using btrfs subvolumes, that's why I'm mounting /dev/sda5 - it's the only btrfs volume in my system. – Piotr Kempa – 2019-03-29T15:06:23.613

1For now I think my only option is to mount a usb drive, copy the files and reformat everything. I know one thing - I'll stick to ext4 this time :) – Piotr Kempa – 2019-03-29T15:07:54.153

I crashed my btrfs too when running out of space. Manjaro forced a multi-Giga update down my SATA cable. I had snaps tho and happily recovered from the crash. otherwise I'd gone back to ext4 too, but now I'm big btrfs fanboi. – dotbit – 2019-10-14T13:13:58.790

No answers

Asked: 2019-03-29T14:42:07.257

Viewed: 362 times

Active: 2019-03-29T15:11:38.557