linux-kernel - Re: [RFC][Resend] Make NFS-Client readahead tunable

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Date:	Wed, 17 Sep 2008 10:06:44 -0400
From:	Peter Staubach <staubach@...hat.com>
To:	Martin Knoblauch <knobi@...bisoft.de>
CC:	linux-nfs list <linux-nfs@...r.kernel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC][Resend] Make NFS-Client readahead tunable

Martin Knoblauch wrote:
> Hi,
>
> the following/attached patch works around a [obscure] problem when an 2.6 (not sure/caring about 2.4) NFS client accesses an "offline" file on a Sun/Solaris-10 NFS server when the underlying filesystem is of type SAM-FS. Happens with RHEL4/5 and mainline kernels. Frankly, it is not a Linux problem, but the chance for a short-/mid-term solution from Sun are very slim. So, being lazy, I would love to get this patch into Linux. If not, I just will have to maintain it for eternity out of tree.
>
> The problem: SAM-FS is Suns proprietary HSM filesystem. It stores meta-data and a relatively small amount of data "online" on disk and pushes old or infrequently used data to "offline" media like e.g. tape. This is completely transparent to the users. If the date for an "offline" file is needed, the so called "stager daemon" copies it back from the offline medium. All of this works great most of the time. Now, if an Linux NFS client tries to read such an offline file, performance drops to "extremely slow". After lengthly investigation of tcp-dumps, mount options and procedures involving black cats at midnight, we found out that the readahead behaviour of the Linux NFS client causes the problem. Basically it seems to issue read requests up to 15*rsize to the server. In the case of the "offline" files, this behaviour causes heavy competition for the inode lock between the NFSD process and the stager daemon on the Solaris server.
>
> - The real solution: fixing SAM-FS/NFSD interaction. Sun engineering acks the problem, but a solution will need time. Lots of it.
> - The working solution: disable the client side readahead, or make it tunable. The patch does that by introducing a NFS module parameter "ra_factor" which can take values between 1 and 15 (default 15) and a tunable "/proc/sys/fs/nfs/nfs_ra_factor" with the same range and default.

Hi.

I was curious if a design to limit or eliminate read-ahead
activity when the server returns EJUKEBOX was considered?
Unless one can know that the server and client can get into
this situation ahead of time, how would the tunable be used?

    Thanx...

       ps

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