Re: Only one subvolume can be mounted after replace/balance

[Chris Murphy <lists@colorremedies.com>]

On Sat, Jan 23, 2021 at 7:50 AM Jakob Schöttl <jschoett@gmail.com> wrote:
>
> Hi,
>
> In short:
> When mounting a second subvolume from a pool, I get this error:
> "mount: /mnt: wrong fs type, bad option, bad superblock on /dev/sda,
> missing code page or helper program, or other."
> dmesg | grep BTRFS only shows this error:
> info (device sda): disk space caching is enabled
> error (device sda): Remounting read-write after error is not allowed

It went read-only before this because it's confused. You need to
unmount it before it can be mounted rw. In some cases a reboot is
needed.

>
> What happened:
>
> In my RAID1 pool with two disk, I successfully replaced one disk with
>
> btrfs replace start 2 /dev/sdx
>
> After that, I mounted the pool and did

I don't understand this sequence. In order to do a replace, the file
system is already mounted.

>
> btrfs fi show /mnt
>
> which showed WARNINGs about
> "filesystems with multiple block group profiles detected"
> (don't remember exactly)
>
> I thought it is a good idea to do
>
> btrfs balance start /mnt
>
> which finished without errors.

Balance alone does not convert block groups to a new profile. You have
to explicitly select a conversion filter, e.g.

btrfs balance start -dconvert=raid1,soft -mconvert=raid1,soft /mnt

> Now, I can only mount one (sub)volume of the pool at a time. Others can
> only be mounted read-only. See error messages at top of this mail.
>
> Do you have any idea what happened or how to fix it?
>
> I already tried rescue zero-log and super-recovery which was successful
> but didn't help.

I advise anticipating the confusion will get worse, and take the
opportunity to refresh the backups. That's the top priority, not
fixing the file system.

Next let us know the following:

kernel version
btrfs-progs version
Output from commands:
btrfs fi us /mnt
btrfs check --readonly


-- 
Chris Murphy