Re: Seed device is broken, again.

[David Sterba <dsterba@suse.cz>]

On Tue, Mar 01, 2022 at 08:13:28AM +0800, Qu Wenruo wrote:
> 
> >
> > There was a discussion some time ago if the log replay should happen on
> > a read-only mount, or any potential repair action that could happen
> > regardless of the ro/rw mount status. The conclusion was that it could
> > happen, and guaranteeing no writes to the block device should be done
> > externally eg. by blockdev --setro. But I think we opted not to do any
> > writes anyway for btrfs.
> 
> My main concern is still there, we change RO to RW without any remount.

We also do emergency remount from RO to RW, so I take it as that
changing the status is partiall in the hands of filesystem, not
violating APIs and such.

> It's weird from the beginning, but we just accept that.
> 
> If we have chance to rethink this, would we still take the same path?
> Other than making seed device into user space tool like mkfs?

I'm not sure it would work the same way as now, the seeding device can
be mounted several times in parallel as the UUID is generated at each
mount, then added the writeable device, then thrown away. Any detour
through userspace make it more complicated, but I haven't thought that
through. We could add it no top of the on-line seeding as another
usecase but if current users are fine with how it works now I don't
think we need to spend time on implementing it.