linux-kernel - Re: [PATCH v4 00/14] forcealign for xfs

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Message-ID: <591f0499-2a5a-437a-be11-453d40169f5c@oracle.com>
Date: Mon, 23 Sep 2024 09:00:07 +0100
From: John Garry <john.g.garry@...cle.com>
To: Dave Chinner <david@...morbit.com>
Cc: Ritesh Harjani <ritesh.list@...il.com>, chandan.babu@...cle.com,
        djwong@...nel.org, dchinner@...hat.com, hch@....de,
        viro@...iv.linux.org.uk, brauner@...nel.org, jack@...e.cz,
        linux-xfs@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, catherine.hoang@...cle.com,
        martin.petersen@...cle.com
Subject: Re: [PATCH v4 00/14] forcealign for xfs

On 23/09/2024 03:57, Dave Chinner wrote:
>> In the meantime, if mkfs auto-enables atomic writes (when the HW supports),
>> what will it do to reflink feature (in terms of enabling)?
> I didn't say we should always "auto-enable atomic writes".
> 
> I said if the hardware is atomic write capable, then mkfs should
> always*align the filesystem* to atomic write constraints.  A kernel
> upgrade will eventually allow reflink and atomic writes to co-exist,
> but only if the filesystem is correctly aligned to the hardware
> constrains for atomic writes. We need to ensure we leave that
> upgrade path open....
> 
> .... and only once we have full support can we make "mkfs
> auto-enable atomic writes".

ok, fine. The current maximum value of atomic write unit max is 512KB 
(assuming 4K PAGE_SIZE and 512B sector size), so that should not be too 
needlessly inefficient for laying out the AGs. However, for 16KB+ 
PAGE_SIZE, that value could naturally be larger. However having HW which 
supports such large atomics would be very unlikely.

Thanks,
John