Re: NFS Still broken in 2.6.x?

[Bryan Fink <bfink@eventmonitor.com>]

Trond Myklebust wrote:

> On Fri, 2006-02-24 at 04:14 -0800, Andrew Morton wrote:
>  
>
> > Trond Myklebust <trond.myklebust@fys.uio.no> wrote:
> >    
> >
> > > On Thu, 2006-02-23 at 15:35 -0500, Bryan Fink wrote:
> > > > Hi All.  I'm running into a bit of trouble with NFS on 2.6.  I see that
> > > > at least Trond thought, mid-January, that "The readahead algorithm has
> > > > been broken in 2.6.x for at least the past 6 months." (
> > > > http://www.ussg.iu.edu/hypermail/linux/kernel/0601.2/0559.html) Anyone
> > > > know if that has been fixed?
> > >
> > > No it hasn't been fixed. ...and no, this is not a problem that only
> > > affects NFS: it just happens to give a more noticeable performance
> > > impact due to the larger latency of NFS over a 100Mbps link.
> > >      
> > iirc, last time we went round this loop Ram and I were unable to reproduce it.
> >
> > Does anyone have a testcase?
> >    
>
> Yes. A dead simple one
>
> run iozone in sequential read mode on a tcp link w/ rsize == 32k
>  
I'm sure Trond's testcase is much more useful, but for reference, I 
thought I'd add that I've been doing my testing with a simple "dd 
if=/nfsmount/file of=/dev/null bs=32k".  /nfsmount/file is usually 2.5-3 
GB, which makes the difference between NFS servers long enough that I 
feel safe throwing a "time" in front of the whole command.  That is, the 
difference is nowhere near millisecond resolution (it's nearer a 
minute), so I like to start the test and then walk away to do other things.

Interesting that it's not an NFS-only bug.  I assumed it was when I 
logged into each server so I could run "dd if=file of=/dev/null bs=32k" 
locally.  When I did that, both servers gave roughly the same speed.  
Sorry I left this bit out of my first email.  I assume this example only 
illustrates how opaque the code around this problem truly is.

Thanks very much for the help.

-Bryan

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