linux-kernel - Re: read-ahead in NFS server

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Date:	Fri, 28 Dec 2007 10:33:21 +0800
From:	Fengguang Wu <wfg@...l.ustc.edu.cn>
To:	saeed bishara <saeed.bishara@...il.com>
Cc:	Jeff Garzik <jeff@...zik.org>, linux-kernel@...r.kernel.org,
	NFS list <linux-nfs@...r.kernel.org>
Subject: Re: read-ahead in NFS server

On Thu, Dec 27, 2007 at 05:00:12PM +0200, saeed bishara wrote:
> > >> Are you using TCP?  Are you using NFSv4, or an older version?
> > > I'm using NFSv3/UDP.
> >
> > IMO, you definitely want TCP and NFSv4.  Much better network behavior,
> > with some of the silly UDP limits (plus greatly improved caching
> > behavior, due to v4 delegations).
> the clients of my system going to be embedded system with low
> performance cpus and I need UDP as it needs less cpu power.

You can try the attached adaptive readahead patch.
Apply it on your server and compile kernel with CONFIG_ADAPTIVE_READAHEAD.
Use large 1MB readahead on server and small readahead on clients.

> > > when I run local dd with bs=4K, I can see that the average IO size is
> > > more than 300KB.
> >
> > Read-ahead is easier in NFSv4, because the client probably has the file
> > delegated locally, and has far less need to constantly revalidate file
> > mapping(s).
> I'll check that.
> but what about the server side? why the issued IO's are only as twice
> as the size of the NFS requests?

The readahead code is helpless in NFSv3 :-(
Use NFS over TCP and rsize=readahead=1MB on client side could help.
But if you prefer UDP, the above patch may help you :-)

Fengguang

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