[OpenAFS-devel] fileserver parameters

[Jeffrey Hutzelman]

On Monday, June 20, 2005 04:55:01 PM +0200 Niklas Edmundsson 
<Niklas.Edmundsson@hpc2n.umu.se> wrote:

> On Mon, 20 Jun 2005, Roland Kuhn wrote:
>
>>> What kernel? What underlying filesystem?
>>>
>> vanilla 2.4.31, reiserfs.
>
> Our experiences of reiserfs are not that great, I would really recommend
> you to try another underlying fs.
>
>>> The performance figures you mentioned when talking directly to the
>>> raid,  are they when doing IO on a filesystem that are similar to the
>>> AFS IO  pattern (ie accessing a large number of small files)? Streaming
>>> IO figures  are more or less uninteresting when doing AFS in our
>>> experience.
>>>
>> I haven't had a look yet, but I thought the files were stored also as
>> files  in /vicep*? Our files are 1.5GB each, so streaming reads should
>> be what we  need, right?
>
> Does AFS store those files as large continous files too? Poke around in
> /vice* and find out (or maybe someone on the list will answer :). I have
> no clue on how the fileserver manages requests too, worst case it would
> do open/seek/read/close for each chunk, but I hope that it's smarter than
> that even though it's a kludge.

The fileserver does store files as large continuous chunks.  And, it caches 
open file handles, so if you are actually reading from a big file at a 
reasonable rate, it shouldn't need to do open/close on every access.  It'll 
need to seek, probably, but that doesn't actually have a performance impact 
on regular files.

However, the cache manager does reads with one RPC per chunk, so normally 
one chunk has to be completely read before you can start asking for the 
next one.  That means you don't get to stream across chunks, which can 
hurt.  You can improve the situation somewhat by using a larger chunk size, 
but that has its own problems.

-- Jeffrey T. Hutzelman (N3NHS) <jhutz+@cmu.edu>
   Sr. Research Systems Programmer
   School of Computer Science - Research Computing Facility
   Carnegie Mellon University - Pittsburgh, PA