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From: Tao Lyu <tao.lyu@epfl.ch>
To: Trond Myklebust <trondmy@hammerspace.com>, "linux-nfs@vger.kernel.org"
	<linux-nfs@vger.kernel.org>
Subject: Re: Question about O_APPEND | O_DIRECT
Thread-Topic: Question about O_APPEND | O_DIRECT
Thread-Index: AQHaHjflhODFfyeMXUSkxSGiPt/2HLCLuRkAgAKTcNA=
Date: Mon, 27 Nov 2023 15:28:16 +0000
Message-ID: <7d2d17e4d3904d29b75fadcfd916b2a3@epfl.ch>
References: <c609e5f9df75438dbfe3810859935d58@epfl.ch>,<2d948b43fa625952e50589e4bedf9551df7ee112.camel@hammerspace.com>
In-Reply-To: <2d948b43fa625952e50589e4bedf9551df7ee112.camel@hammerspace.com>
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>> Hi,
>>=20
>> Sorry to bother you here.
>>=20
>> I'm using NFS and realize it doesn't support opening a file with
>> "O_DIRECT | O_APPEND".
>>=20
>> After checking the source code,=20
>> I found it has one function that checks explicitly whether there is a
>> combination flag of "O_APPEND | O_DIRECT".
>> If so, it will return invalid arguments.
>>=20
>> int nfs_check_flags(int flags)
>> {
>> =A0=A0=A0 if ((flags & (O_APPEND | O_DIRECT)) =3D=3D (O_APPEND | O_DIREC=
T))
>> =A0=A0=A0=A0=A0=A0=A0 return -EINVAL;
>>=20
>> =A0=A0=A0 return 0;
>> }
>>=20
>> But I don't understand why NFS doesn't support this flag combination.
>> I'd appreciate it if someone could explain this to me.
>
>
> Why do you need O_APPEND|O_DIRECT?
>
> In order to implement O_APPEND|O_DIRECT, we would need to add an APPEND
> operation, which does not exist in the NFS protocol. The WRITE
> operation does not suffice, because it requires you to know the offset
> at which you will be writing the data.

Hi Trond,

Thank you so much for your reply.

O_APPEND | O_DIRECT can be used to bypass the client cache for multiple thr=
eads 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.

Thank you in advance for helping me.

Best,
Tao