Re: [PATCH 0/2] Supporting same fsid filesystems mounting on btrfs

[Goffredo Baroncelli <kreijack@inwind.it>]

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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