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From:   "Benjamin Coddington" <bcodding@redhat.com>
To:     "J. Bruce Fields" <bfields@fieldses.org>
Cc:     "Olga Kornievskaia" <aglo@umich.edu>,
        linux-nfs <linux-nfs@vger.kernel.org>
Subject: Re: unsharing tcp connections from different NFS mounts
Date:   Wed, 07 Oct 2020 07:27:26 -0400
Message-ID: <57E3293C-5C49-4A80-957B-E490E6A9B32E@redhat.com>
In-Reply-To: <20201007001814.GA5138@fieldses.org>
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 <20201007001814.GA5138@fieldses.org>
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On 6 Oct 2020, at 20:18, J. Bruce Fields wrote:

> On Tue, Oct 06, 2020 at 05:46:11PM -0400, Olga Kornievskaia wrote:
>> On Tue, Oct 6, 2020 at 3:38 PM Benjamin Coddington 
>> <bcodding@redhat.com> wrote:
>>>
>>> On 6 Oct 2020, at 11:13, J. Bruce Fields wrote:
>>>
>>>> NFSv4.1+ differs from earlier versions in that it always performs
>>>> trunking discovery that results in mounts to the same server 
>>>> sharing a
>>>> TCP connection.
>>>>
>>>> It turns out this results in performance regressions for some 
>>>> users;
>>>> apparently the workload on one mount interferes with performance of
>>>> another mount, and they were previously able to work around the
>>>> problem
>>>> by using different server IP addresses for the different mounts.
>>>>
>>>> Am I overlooking some hack that would reenable the previous 
>>>> behavior?
>>>> Or would people be averse to an "-o noshareconn" option?
>>>
>>> I suppose you could just toggle the nfs4_unique_id parameter.  This
>>> seems to
>>> work:
>>>
>>> flock /sys/module/nfs/parameters/nfs4_unique_id bash -c 
>>> "OLD_ID=\$(cat
>>> /sys/module/nfs/parameters/nfs4_unique_id); echo imalittleteapot >
>>> /sys/module/nfs/parameters/nfs4_unique_id; mount -ov4,sec=sys
>>> 10.0.1.200:/exports /mnt/fedora2; echo \$OLD_ID >
>>> /sys/module/nfs/parameters/nfs4_unique_id"
>>>
>>> I'm trying to think of a reason why this is a bad idea, and not 
>>> coming
>>> up
>>> with any.  Can we support users that have already found this 
>>> solution?
>>>
>>
>> What about reboot recovery? How will each mount recover its own state
>> (and present the same identifier it used before). Client only keeps
>> track of one?
>
> Looks like nfs4_init_{non}uniform_client_string() stores it in
> cl_owner_id, and I was thinking that meant cl_owner_id would be used
> from then on....
>
> But actually, I think it may run that again on recovery, yes, so I bet
> changing the nfs4_unique_id parameter midway like this could cause 
> bugs
> on recovery.

Ah, that's what I thought as well.  Thanks for looking closer Olga!

I don't see why we couldn't store it for the duration of the mount, and
doing so would fix reboot recovery when the uniquifier is changed after 
a
mount.

Ben