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

["Jakob Schöttl" <jschoett@gmail.com>]

Thank you Chris, it's resolved now, see below.

Am 25.01.21 um 23:47 schrieb Chris Murphy:
> 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.
Oh, I didn't notice that the pool was already mounted (via fstab).
The filesystem where out of space and I had to resize both disks 
separately. And I had to mount with -o skip_balance for that. Now it 
works again.

>> 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.
That was, what I did before my actual problem occurred. But it's 
resolved now.

>> 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
I didn't want to convert to a new profile. I thought btrfs replace 
automatically uses the same profile as the pool?

Regards, Jakob