On Wed, Jan 27, 2021 at 6:10 AM Jakob Schöttl [off-list ref] wrote:

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

Am 25.01.21 um 23:47 schrieb Chris Murphy:

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On Sat, Jan 23, 2021 at 7:50 AM Jakob Schöttl [off-list ref] wrote:

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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.

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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.

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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?

Btrfs replace does not change the profile. But you reported mixed
profile block groups, which means conversion is indicated to make sure
they're al the same. Please post:

sudo btrfs fi us /mnt

Let's see what the block groups are and what you want them to be and
then see what conversion command might be indicated.


-- 
Chris Murphy