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Subject: Re: Spurious instability with NFSoRDMA under moderate load
To:     Chuck Lever III <chuck.lever@oracle.com>
Cc:     Linux NFS Mailing List <linux-nfs@vger.kernel.org>,
        Olga Kornievskaia <aglo@umich.edu>,
        Dai Ngo <dai.ngo@oracle.com>
References: <4da3b074-a6be-d83f-ccd4-b151557066aa@rothenpieler.org>
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From:   Timo Rothenpieler <timo@rothenpieler.org>
Message-ID: <97fe445e-6c4f-b7cf-fa39-fd0c4222a89e@rothenpieler.org>
Date:   Tue, 17 Aug 2021 23:51:57 +0200
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On 17.08.2021 23:08, Chuck Lever III wrote:
> I tried reproducing this using your 'xfs_io -fc "copy_range
> testfile" testfile.copy' reproducer, but couldn't.
>=20
> A network capture shows that the client tries CLONE first. The
> server reports that's not supported, so the client tries COPY.
> The COPY works, and the reply shows that the COPY was synchro-
> nous. Thus there's no need for a callback, and I'm not tripping
> over backchannel misbehavior.

Make sure the testfile is of sufficient size. I'm not sure what the=20
threshold is, but if it's too small, it'll just do a synchronous copy=20
for me as well.
I'm using a 50MB file in my tests.

> The export I'm using is an xfs filesystem. Did you already
> report the filesystem type you're testing against? I can't
> find it in the thread.
>=20
> If there's a way to force an offload-style COPY, let me know.
>=20
> Oh. Also I looked at what might have been pulled into the
> linux-5.12.y kernel between .12 and .19, and I don't see
> anything that's especially relevant to either COPY_OFFLOAD
> or backchannel.

I'm observing this with both an ext4 and zfs filesystem.
Can easily test xfs as well if desired.

Are you testing this on a normal network, or with RDMA? With normal tcp, =

I also can't observe this issue(it doesn't time out the backchannel in=20
the first place), it only happens in RDMA mode.
I'm using Mellanox ConnectX-4 cards in IB mode for my tests.


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