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

[John Garry <john.g.garry@oracle.com>]

On 07/01/2025 17:13, Mikulas Patocka wrote:
> 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.

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.

> 
> 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