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

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Date:	Wed, 17 Sep 2008 10:41:15 -0500
From:	"Chuck Lever" <chucklever@...il.com>
To:	"Peter Staubach" <staubach@...hat.com>
Cc:	"Martin Knoblauch" <knobi@...bisoft.de>,
	"linux-nfs list" <linux-nfs@...r.kernel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC][Resend] Make NFS-Client readahead tunable

On Wed, Sep 17, 2008 at 9:06 AM, Peter Staubach <staubach@...hat.com> wrote:
> 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?

I tend to agree.  A tunable is probably not a good solution in this case.

I would bet that this lock contention issue is a problem in other more
common cases, and would merit some careful analysis.

-- 
Chuck Lever
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