Re: [PATCH RFC 0/5] device mapper atomic write support

[Mikulas Patocka <mpatocka@redhat.com>]

On Tue, 7 Jan 2025, John Garry wrote:

> On 07/01/2025 17:13, Mikulas Patocka wrote:
> > On Mon, 6 Jan 2025, John Garry wrote:
> > 
> > BTW. could it be possible to add dm-mirror support as well? dm-mirror is
> > used when the user moves the logical volume to another physical volume, so
> > it would be nice if this worked without resulting in not-supported errors.
> > 
> > dm-mirror uses dm-io to perform the writes on multiple mirror legs (see
> > the function do_write() -> dm_io()), I looked at the code and it seems
> > that the support for atomic writes in dm-mirror and dm-io would be
> > straightforward.
> 
> FWIW, we do support atomic writes for md raid1. The key principle is that we
> atomically write to each disk. Obviously we cannot write to multiple disks
> atomically. So the copies in each mirror may be out-of-sync after an
> unexpected power fail, but that is ok as either will have all of old or new
> data, which is what we guarantee.

Yes - something like that can be implemented for dm-mirror too.

> > Another possibility would be dm-snapshot support, assuming that the atomic
> > i/o size <= snapshot chunk size, the support should be easy - i.e. just
> > pass the flag REQ_ATOMIC through. Perhaps it could be supported for
> > dm-thin as well.
> 
> Do you think that there will be users for these?
> 
> atomic writes provide guarantees for users, and it would be hard to detect
> when these guarantees become broken through software bugs. I would be just
> concerned that we enable atomic writes for many of these more complicated
> personalities, and they are not actively used and break.
> 
> Thanks,
> John

dm-snapshot is not much used, but dm-thin is. I added Joe to the 
recipients list, so that he can decide whether dm-thin should support 
atomic writes or not.

Mikulas