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

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Message-ID: <dcbaadea-66c1-4d98-8a37-945d8b336d5b@oracle.com>
Date: Mon, 6 Jan 2025 18:14:38 +0000
From: John Garry <john.g.garry@...cle.com>
To: Mike Snitzer <snitzer@...merspace.com>
Cc: axboe@...nel.dk, agk@...hat.com, hch@....de, mpatocka@...hat.com,
        martin.petersen@...cle.com, linux-block@...r.kernel.org,
        dm-devel@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC 0/5] device mapper atomic write support

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.

> I wrote and/or fixed a fair
> amount of the non-atomic block limits stacking code over the
> years.. so this is just me trying to inform this effort based on
> limits stacking gotchas we've experienced to this point.

Yeah, understood. And that is why I am on the lookup for points at which 
we try to split atomic writes in the submission patch. The only reason 
that it should happen is due to the limits being incorrectly calculated.

> 
> Looks like adding dm-striped support would just need to ensure that
> the chunk_size is multiple of atomic write size (so chunk_size >=
> atomic write size).

Right, so the block queue limits code already will throttle the atomic 
write max so that chunk_size % atomic write upper limit == 0.

> 
> Relative to linear, testing limits stacking in terms of linear should
> also verify that concatenated volumes work.

ok,

Thanks,
John