Disable NFS client cache (or other traffic reduction methods)

[Mohan Srinivasan]

Hi,

Unfortunately, there's no way in the FreeBSD NFS client to 
disable data caching. Setting the ac* values to 0 will have
the effect of disabling the attribute cache, which means 
that you'll never fetch attributes from the cache, but force
an over-the-wire getattr call each time. But this does not
disable data caching. For reference, take a look at 
nfs_loadattrcache() and nfs_getattrcache().

The real reason for tweaking the ac* values is to improve
on the consistency of the cached data (for applications with
data shared across multiple clients). By setting the ac* values
to 0 and bypassing attr caching completely, you force much 
more frequent mtime checks giving you much better consistency
of the cached data.

I added Direct IO support for the NFS client for exactly this
reason - to bypass data caching. But I tickled a bug in the 
-current NFS server when I use Direct IO on the client and 
I have left it disabled until I get a chance to fix that bug.
Direct IO support works fine when used against a FreeBSD 4.x
NFS server or a NetApp filer.

mohan

--- Eli Dart <dart at nersc.gov> wrote:
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> 
> >> Is there a way to disable client-side caching?  I've looked, and
> >> can't seem to find one.  Or, do I have this wrong and there is
> >> something else that is causing this?
> > 
> > 
> > Take a look through "man mount_nfs" for:
> > 
> >      -a      Set the read-ahead count to the specified value.  This may
> > be in
> >              the range of 0 - 4, and determines how many blocks will be
> > read
> >              ahead when a large file is being read sequentially.  Trying a
> >              value greater than 1 for this is suggested for mounts with a
> >              large bandwidth * delay product.
> 
> I did see this.  Since the client is not reading any files (I think
> all the reads are being done by the NFS layer, not by any userland
> processes with open files) I had ignored -a.
> 
> > 
> > ...and the -o ac*** options.  Although you may be right-- I was a bit
> > surprised not to see a "-o noac", myself.  Solaris has one:
> 
> Hmmmm.....any NFS gurus know whether setting the ac* options to 0
> will disable client caching?  Or, will setting these to 0 result in
> the client NFS layer fetching the data from the server and then
> immediately expiring it?
> 
> Thanks,
> 
> 		--eli
> 
> 
> 
> > 
> >            noac  Suppress data and attribute caching.
> > 
> > :-)
> > 
> 
> - --
> 
> - -------------------------------------------------------------------
> Eli Dart                                     Office: (510) 495-2999
> NERSC Center Networking and Security Group   Cell:   (510) 703-4508
> Lawrence Berkeley National Laboratory        Fax:    (510) 486-4316
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