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

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Message-ID: <c8a9dba5-7d02-4aa2-a01f-dd7f53b24938@oracle.com>
Date: Mon, 16 Sep 2024 11:24:56 +0100
From: John Garry <john.g.garry@...cle.com>
To: Dave Chinner <david@...morbit.com>, Ritesh Harjani <ritesh.list@...il.com>
Cc: 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 16/09/2024 08:03, Dave Chinner wrote:
> OTOH, we can't do this with atomic writes. Atomic writes require
> some mkfs help because they require explicit physical alignment of
> the filesystem to the underlying storage. 

If we are enabling atomic writes at mkfs time, then we can ensure agsize 
% extsize == 0. That provides the physical alignment guarantee. It also 
makes sense to ensure extsize is a power-of-2.

However, extsize is re-configurble per inode. So, for an inode enabled 
for atomic writes, we must still ensure agsize % new extsize == 0 (and 
also new extsize is a power-of-2)

> Hence we'll eventually end
> up with atomic writes needing to be enabled at mkfs time, but force
> align will be an upgradeable feature flag.

Could atomic writes also be an upgradeable feature? We just need to 
ensure that agsize % extsize == 0 for an inode enabled for atomic 
writes. Valid extsize values may be quite limited, though, depending on 
the value of agsize.

Thanks,
John