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

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Date:	Thu, 18 Sep 2008 18:45:01 +1000
From:	Greg Banks <gnb@...bourne.sgi.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:
> ----- Original Message ----
>
>   
>>
>> I think having a tunable for client readahead is an excellent idea,
>> although not to solve your particular problem.  The SLES10 kernel has a
>> patch which does precisely that, perhaps Neil could post it.
>>
>> I don't think there's a lot of point having both a module parameter and
>> a sysctl.
>>
>>     
>
>  Actually there is a good reason. The module parameter can be used to set the new value at load time and never bother again. The sysctl is very convenient when doing experiments.
>   
You can set module parameters after module load in
/sys/module/$module/parameters.
>  As Andrew already pointed out, the best solution would be a mount option.
Yep.
>  But that seems much more involved as my workaround patch.
>
>   
Yep.
>> A maximum of 15 is unwise.  I've found that (at least with the older
>> readahead mechanisms in SLES10) a multiple of 4 is required to preserve
>> rsize-alignment of READ rpcs to the server, which helps a lot with wide
>> RAID backends.  So in SGI we tune client readahead to 16.
>>
>>     
>
>  15 is the value that the Linux NFS client uses., at least since 2.6.3. 
It's a silly value.
> As it is not tunable up to today, the comment seems moot :-)  But it opens the questions:
>
> a) should 1 be the minimum, or 0?
>   
Turning off client RA entirely is potentially useful.
> b) can the backing_dev_info.ra_pages field safely be set to something higher than 15?
>   
Yes.  Did I mention 16 ?
>   
>> Your patch seems to have a bunch of other unrelated stuff mixed in.
>>
>>     
>
>  Yeah, someone already pointed out, that the Makefile hunk does not belong there. But you say "a bunch" - anything else?
>   
I rapidly scrolled past some stuff about 64bit inodes.
> Cheers
> Martin
> PS: Did we ever meet/mail when I was at SGI (1991-1997)?
>
>   
I'm more recent than that.

-- 
Greg Banks, P.Engineer, SGI Australian Software Group.
Be like the squirrel.
I don't speak for SGI.

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