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From: Chuck Lever <chuck.lever@oracle.com>
To: Anna Schumaker <schumaker.anna@gmail.com>
Cc: Trond.Myklebust@hammerspace.com,
	Linux NFS Mailing List <linux-nfs@vger.kernel.org>
Subject: Re: [PATCH v2 4/6] NFS: Add READ_PLUS data segment support
Date: Thu, 20 Feb 2020 13:54:15 -0500
Message-ID: <F1B830F5-882A-468A-88C4-6015DB557376@oracle.com> (raw)
In-Reply-To: <2c46295c6a6e748947af226dc470f7e35a2c6e82.camel@gmail.com>


> On Feb 20, 2020, at 1:35 PM, Anna Schumaker <schumaker.anna@gmail.com> wrote:
> 
> On Thu, 2020-02-20 at 13:30 -0500, Chuck Lever wrote:
>>> On Feb 20, 2020, at 1:28 PM, Anna Schumaker <schumaker.anna@gmail.com>
>>> wrote:
>>> 
>>> On Thu, 2020-02-20 at 09:55 -0500, Chuck Lever wrote:
>>> 
>>>> The down-side here is that would make NFSv4.2 on RDMA
>>>> unable to recognize holes in files the same way as it
>>>> does on TCP, and that's a pretty significant variation
>>>> in behavior. Does "noreadplus" even deal with that?
>>> 
>>> Setting "noreadplus" just causes the client to use the READ operation
>>> instead,
>>> so there should be no difference between v4.1 and v4.2 if the option is set.
>> 
>> My concern is the difference between NFSv4.2 with noreadplus
>> and NFSv4.2 with readplus. The former is not able to detect
>> holes in files on the server, but the latter is.
> 
> The client could still use lseek to detect holes. The client throws away the
> hole information after xdr decoding, and zeroes out the corresponding pages for
> the page cache.

Then maybe that's an optimization for another day. The READ_PLUS
reply can have hole information. The client could save itself some
SEEK_HOLE operations if it cached that hole information somehow.

But if "readplus" and "noreadplus" result in exactly the same hole
retention behavior on our client, I guess there's no harm in using
"noreadplus" instead, except for possible performance regression.

An alternative to making "noreadplus" the default would be to
temporarily add "noreadplus" semantics to the "proto=rdma" mount
option, as a separate patch; maybe after some performance results
show that it is necessary. That would keep the mount interface a
little less complicated.


--
Chuck Lever




  reply index

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-14 21:12 [PATCH v2 0/6] NFS: Add support for the v4.2 READ_PLUS operation schumaker.anna
2020-02-14 21:12 ` [PATCH v2 1/6] SUNRPC: Split out a function for setting current page schumaker.anna
2020-02-14 21:12 ` [PATCH v2 2/6] SUNRPC: Add the ability to expand holes in data pages schumaker.anna
2020-02-14 21:12 ` [PATCH v2 3/6] SUNRPC: Add the ability to shift data to a specific offset schumaker.anna
2020-02-14 21:12 ` [PATCH v2 4/6] NFS: Add READ_PLUS data segment support schumaker.anna
2020-02-14 22:28   ` Chuck Lever
2020-02-20 14:42     ` Anna Schumaker
2020-02-20 14:55       ` Chuck Lever
2020-02-20 18:28         ` Anna Schumaker
2020-02-20 18:30           ` Chuck Lever
2020-02-20 18:35             ` Anna Schumaker
2020-02-20 18:54               ` Chuck Lever [this message]
2020-02-14 21:12 ` [PATCH v2 5/6] NFS: Add READ_PLUS hole segment decoding schumaker.anna
2020-02-14 21:12 ` [PATCH v2 6/6] NFS: Decode multiple READ_PLUS segments schumaker.anna
2020-02-20 17:40 ` [PATCH v2 0/6] NFS: Add support for the v4.2 READ_PLUS operation Chuck Lever
2020-02-20 18:25   ` Anna Schumaker
2020-02-20 18:29     ` Chuck Lever

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