[OpenAFS] openafs on Fedora 12?

Rainer Toebbicke rtb@pclella.cern.ch
Fri, 11 Dec 2009 13:41:13 +0100

Simon Wilkinson wrote:

> As you note, our Linux implementation creates two copies of the data - 
> one in AFS's mapping, the other in the backing files. However, we cannot 
> easily get rid of this duplication - there's no simple mechanism of 
> bypassing the VM and 'writing into the chunk files directly'. Using 
> direct-IO would be a possibility, but we'd need to handle doing this in 
> the backgound, otherwise the user would end up having to wait until 
> chunk files actually made it to the disk, and it would limit the range 
> of filesystems we can use as a backing cache.


the afs that we distribute (as part of SLC - Scientific Linux for CERN) works 
with chunk files directly from the afs_linux_{read,write} entry points, namely 
in order to avoid the double copy and footprint.

We only go through the VM when the file gets mmapped (nopage/readpage), which 
except for 'ld' and running binaries does not happen very often.

Hence, caching still happens but on the chunk file level.

We've been running that code for some 3-4 years now, alas I never got around 
to benchmarking it properly. Apart from very small buffer sizes I don't see 
why avoiding another copy could slow things down, though. I was about to 
submit it to openafs.org once when I discovered that I had accidentally messed 
up the read-ahead. And then forgot again...

It's for 1.4, though. I'd have to look at 1.5.

Rainer Toebbicke
European Laboratory for Particle Physics(CERN) - Geneva, Switzerland
Phone: +41 22 767 8985       Fax: +41 22 767 7155