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

[Chuck Lever]

On Wed, Sep 17, 2008 at 11:23 AM, Martin Knoblauch <knobi@xxxxxxxxxxxx> wrote:
> ----- Original Message ----
>
>> From: Chuck Lever <chucklever@xxxxxxxxx>
>> To: Peter Staubach <staubach@xxxxxxxxxx>
>> Cc: Martin Knoblauch <knobi@xxxxxxxxxxxx>; linux-nfs list <linux-nfs@xxxxxxxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx
>> Sent: Wednesday, September 17, 2008 5:41:15 PM
>> Subject: Re: [RFC][Resend] Make NFS-Client readahead tunable
>>
>> On Wed, Sep 17, 2008 at 9:06 AM, Peter Staubach 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.
>>
>
>  Are you talking wrt. a Solaris NFS-Server with SAM-FS/QFS as backend filesystem?

I misread your mail, and thought the inode lock contention issue was
on the client.

--
Chuck Lever
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