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Date: Mon, 27 Nov 2023 09:19:35 -0800
From: "hch@infradead.org" <hch@infradead.org>
To: Trond Myklebust <trondmy@hammerspace.com>
Cc: "hch@infradead.org" <hch@infradead.org>,
	"tao.lyu@epfl.ch" <tao.lyu@epfl.ch>,
	"linux-nfs@vger.kernel.org" <linux-nfs@vger.kernel.org>
Subject: Re: Question about O_APPEND | O_DIRECT
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On Mon, Nov 27, 2023 at 05:08:22PM +0000, Trond Myklebust wrote:
> Note that APPEND would only really work with O_DIRECT, since it is
> anathema to cached I/O to not be able to control the placement of the
> data.

Yes.

> In addition, the model will always break down if someone decides they
> want to write a log entry of size > wsize. Once you have to split up
> the data, you (obviously) lose the atomicity you need in order to write
> a contiguous record.

Yes.  Note that there is work going on to define atomic I/O limits
on the various Linux lists currently.  Although in the block layer
we also have a separate limit for the maximum append size already.