On 04/05/2023 22.10, Guilherme G. Piccoli wrote:
On 04/05/2023 16:28, Goffredo Baroncelli wrote:
[...]
Hi Guilherme,
did you tried to run "btrfs dev scan --forget /dev/sd.." before
mount the filesystem ?
Assuming that you have two devices /dev/sdA and /dev/sdB with two btrfs
filesystem with the same uuid, you should mount /dev/sdA
btrfs dev scan --forget /dev/sdB # you can use event /dev/sdA
mount /dev/sdA /mnt/target
and to mount /dev/sdB
btrfs dev scan --forget /dev/sdA # you can use event /dev/sdB
mount /dev/sdB /mnt/target
I made a quick test using two loop devices and it seems that it works
reliably.
Hi Goffredo, thanks for your suggestion!
This seems interesting with regards the second patch here..indeed, I can
mount any of the 2 filesystems if I use the forget option - interesting
option, wasn't aware of that.
But unfortunately it seems limited to mounting one device at a time, and
we need to be able to mount *both* of them, due to an installation step.
If I try to forget the device that is mounted, it gets (obviously) a
"busy device" error.
Ahh, I didn't realized that you want to mount the two FS at the
same time.
Is there any missing step from my side, or mounting both devices is
really a limitation when using the forget option?
From my limited BTRFS internal knowledge, I think that your patches
takes the correct approach: using the "metadata_uuid" code to allow
two filesystems with the same uuid to exist at the same time.
Another option should be make a kernel change that "forget" the device
before mounting *if* the filesystem is composed by only one device (
and another few exceptions like the filesystem is already mounted).
This would avoid all the problem related to make a "temporary" uuid.
I guess again this would be useful in the scope of the second patch
here...we could check the way you're proposing instead of having the
module parameter. In a way this is similar to the forget approach,
right? But it's kind of an "automatic" forget heh
How btrfs would know it is a case for single-device filesystem? In other
words: how would we distinguish between the cases we want to auto-forget
before mounting, and the cases in which this behavior is undesired?
If I remember correctly in the super-block there is the number of disks
that compose the filesystem. If the count is 1, it should be safe to
forget-before-mount the filesystem (or to not store in the cache
after a scan)
Thanks again for your feedback, it is much appreciated.
Cheers,
Guilherme
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