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Date: Tue, 1 Oct 2024 10:43:02 +0200
From: Christoph Hellwig <hch@lst.de>
To: "Darrick J. Wong" <djwong@kernel.org>
Cc: John Garry <john.g.garry@oracle.com>, axboe@kernel.dk,
	brauner@kernel.org, viro@zeniv.linux.org.uk, jack@suse.cz,
	dchinner@redhat.com, hch@lst.de, cem@kernel.org,
	linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	hare@suse.de, martin.petersen@oracle.com,
	catherine.hoang@oracle.com, mcgrof@kernel.org,
	ritesh.list@gmail.com, ojaswin@linux.ibm.com
Subject: Re: [PATCH v6 5/7] xfs: Support atomic write for statx
Message-ID: <20241001084302.GD20648@lst.de>
References: <20240930125438.2501050-1-john.g.garry@oracle.com> <20240930125438.2501050-6-john.g.garry@oracle.com> <20240930163716.GO21853@frogsfrogsfrogs>
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On Mon, Sep 30, 2024 at 09:37:16AM -0700, Darrick J. Wong wrote:
> Ok, so we're only supporting untorn writes if they're exactly the fs
> blocksize, and 1 fsblock is between awu_min/max.  That simplifies a lot
> of things. :)
> 
> Not supporting sub-fsblock atomic writes means that we'll never hit the
> directio COW fallback code, which uses the pagecache.
> 
> Not supporting multi-fsblock atomic writes means that you don't have to
> figure out how to ensure that we always do cow on forcealign
> granularity.  Though as I pointed out elsewhere in this thread, that's a
> forcealign problem.

It does simplify things a lot, and is probably a good idea for
the initial version.  But I suspect support for atomic writes
smaller than the block size will be really useful and we should
eventually support them, but maybe now on reflinked files or
alwayscow.