linux-kernel - Re: [PATCH 17/21] fs: xfs: iomap atomic write support

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Date:   Fri, 10 Nov 2023 10:42:37 +0000
From:   John Garry <john.g.garry@...cle.com>
To:     Christoph Hellwig <hch@....de>
Cc:     axboe@...nel.dk, kbusch@...nel.org, sagi@...mberg.me,
        jejb@...ux.ibm.com, martin.petersen@...cle.com, djwong@...nel.org,
        viro@...iv.linux.org.uk, brauner@...nel.org,
        chandan.babu@...cle.com, dchinner@...hat.com,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, tytso@....edu, jbongio@...gle.com,
        linux-api@...r.kernel.org
Subject: Re: [PATCH 17/21] fs: xfs: iomap atomic write support

On 09/11/2023 15:26, Christoph Hellwig wrote:
> On Fri, Sep 29, 2023 at 10:27:22AM +0000, John Garry wrote:
>> Ensure that when creating a mapping that we adhere to all the atomic
>> write rules.
>>
>> We check that the mapping covers the complete range of the write to ensure
>> that we'll be just creating a single mapping.
>>
>> Currently minimum granularity is the FS block size, but it should be
>> possibly to support lower in future.
> I really dislike how this forces aligned allocations.  Aligned
> allocations are a nice optimization to offload some of the work
> to the storage hard/firmware, but we need to support it in general.
> And I think with out of place writes into the COW fork, and atomic
> transactions to swap it in we can do that pretty easily.
> 
> That should also allow to get rid of the horrible forcealign mode,
> as we can still try align if possible and just fall back to the
> out of place writes.
> 
> 

How could we try to align? Do you mean that we try to align up to some 
stage in the block allocator search? That seems like some middle ground 
between no alignment and forcealign.

And what would we be aligning to?

Thanks,
John