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Subject: Re: [PATCH 0/9] Multiple network connections for a single NFS mount.
To:     NeilBrown <neilb@suse.com>, Olga Kornievskaia <aglo@umich.edu>,
        Chuck Lever <chuck.lever@oracle.com>,
        Schumaker Anna <Anna.Schumaker@netapp.com>,
        Trond Myklebust <trondmy@hammerspace.com>
Cc:     linux-nfs@vger.kernel.org
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From:   Tom Talpey <tom@talpey.com>
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Date:   Thu, 30 May 2019 13:05:04 -0400
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On 5/29/2019 8:41 PM, NeilBrown wrote:
> I've also re-arrange the patches a bit, merged two, and remove the
> restriction to TCP and NFSV4.x,x>=1.  Discussions seemed to suggest
> these restrictions were not needed, I can see no need.

I believe the need is for the correctness of retries. Because NFSv2,
NFSv3 and NFSv4.0 have no exactly-once semantics of their own, server
duplicate request caches are important (although often imperfect).
These caches use client XID's, source ports and addresses, sometimes
in addition to other methods, to detect retry. Existing clients are
careful to reconnect with the same source port, to ensure this. And
existing servers won't change.

Multiple connections will result in multiple source ports, and possibly
multiple source addresses, meaning retried client requests may be
accepted as new, rather than having any chance of being recognized as
retries.

NFSv4.1+ don't have this issue, but removing the restrictions would
seem to break the downlevel mounts.

Tom.