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

[Mikulas Patocka <mpatocka@redhat.com>]

On Mon, 6 Jan 2025, John Garry wrote:

> On 06/01/2025 17:26, Mike Snitzer wrote:
> > On Mon, Jan 06, 2025 at 12:41:14PM +0000, John Garry wrote:
> > > This series introduces initial device mapper atomic write support.
> > > 
> > > Since we already support stacking atomic writes limits, it's quite
> > > straightforward to support.
> > > 
> > > Only dm-linear is supported for now, but other personalities could
> > > be supported.
> > > 
> > > Patch #1 is a proper fix, but the rest of the series is RFC - this is
> > > because I have not fully tested and we are close to the end of this
> > > development cycle.
> > In general, looks reasonable.  But I would prefer to see atomic write
> > support added to dm-striped as well.  Not that I have some need, but
> > because it will help verify the correctness of the general stacking
> > code changes (in both block and DM core). 
> 
> That should be fine. We already have md raid0 support working (for atomic
> writes), so I would expect much of the required support is already available.

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.

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.

Mikulas