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

[Martin Knoblauch]

----- Original Message ----

> From: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> To: Martin Knoblauch <knobi@xxxxxxxxxxxx>
> Cc: Greg Banks <gnb@xxxxxxxxxxxxxxxxx>; linux-nfs list <linux-nfs@xxxxxxxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx
> Sent: Thursday, September 18, 2008 10:18:18 AM
> Subject: Re: [RFC][Resend] Make NFS-Client readahead tunable
> 
> On Thu, 18 Sep 2008 00:42:58 -0700 (PDT) Martin Knoblauch 
> wrote:
> 
> > ----- Original Message ----
> > 
> > > From: Andrew Morton 
> > > To: Greg Banks 
> > > Cc: Martin Knoblauch ; linux-nfs list 
> ; linux-kernel@xxxxxxxxxxxxxxx
> > > Sent: Thursday, September 18, 2008 5:13:34 AM
> > > Subject: Re: [RFC][Resend] Make NFS-Client readahead tunable
> > > 
> > > On Thu, 18 Sep 2008 11:42:54 +1000 Greg Banks wrote:
> > > 
> > > > 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.
> > > 
> > > mount -o remount,readahead=42
> > 
> > [root@lpsdm52 ~]# mount -o remount,readahead=42 /net/spsdms/fs13
> > Bad nfs mount parameter: readahead
> > [root@lpsdm52 ~]# mount -o readahead=42 /net/spsdms/fs13
> > Bad nfs mount parameter: readahead
> > 
> > 
> >  I assume the reply was meant to say that the correct way of introducing a 
> modifyable readahead size is to implement it as a mount option ? :-)
> 
> Yes.
>

:-)
 
> > I considered it, but it seems to be more intrusive than the workaround patch. 
> It also needs changes to userspace tools - correct?
> 
> No.  mount(8) will pass unrecognised options straight down into the
> filesystem driver.
>

 Has that always been the case, or is it a recent change? I have to support RHEL4 userland, which is not really new.
 
> It's better this way - it allows the tunable to be set on a per-mount
> basis rather than machine-wide.
>

 No question about that. I just thought it to be to complicated. Maybe I erred.
 
> Note that for block devices, readahead is a per-backing_dev_info thing
> (and a backing_dev_info has a 1:1 relationship to a disk drive for sane
> setups).  
> 
> And the NFS client maintains a backing_dev_info, which appears to map
> onto a server, so making the NFS readahead a per-backing_dev_info (ie:
> per server) thing might make sense.  Maybe nfs makes per-server information
> manipulatable down in sysfs somewhere..

 I believe Peter wanted to add per bdi stuff for nfs some time ago. Not sure what came out of it.

Cheers
Martin
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