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Date: Mon, 27 Nov 2023 08:36:47 -0800
From: Christoph Hellwig <hch@infradead.org>
To: Tao Lyu <tao.lyu@epfl.ch>
Cc: Trond Myklebust <trondmy@hammerspace.com>,
	"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 03:28:16PM +0000, Tao Lyu wrote:
> 
> O_APPEND | O_DIRECT can be used to bypass the client cache for multiple threads writing data without caring of the orders (e.g., logs).
> 
> Yes, to support O_APPEND | O_DIRECT, NFS must first support APPEND.
> But the key point is that looks like NFS has supported O_APPEND already.
> I can successfully open a file with "O_RDWR|O_APPEND".
> 
> My confusion is why NFS supports O_RDWR and O_APPEND individually but does not support this combination.

Well, it does support O_RDWR|O_APPEND, just not with O_DIRECT?

Btw, I think an APPEND operation in NFS would be a very good idea, and
I'd love to work with interested parties in the IETF on it.  Not that
we (Damien to be specific) plan to add support to Linux to also report
the actual offset an O_APPEND write wrote to through io_uring as we
have varios use cases for out of place write data stores for that.
It would be great to also support that programming model over NFS.