[OpenAFS] Re: Linux tmpfs

Rainer Toebbicke rtb@pclella.cern.ch
Wed, 19 Nov 2008 10:18:17 +0100

> Does the disk cache take advantage of linux's disk cache? I'm thinking
> that is a freebie if we use tmpfs or some other linux-provided fs-backed
> cache.

Yes, it does, it just reads cache files like any vanilla user space process.

This has two consequences:

1. tmpfs on linux just works fine, if you have a (small) patch that glues the 
inode-centric file opens to the dentry-centric tmpfs files. I suspect the work 
done to make OSX happy obsoletes this patch if it can be made to work under Linux.

Actually, since tmpfs's disk backing can span disks this would be an 
attractive option to speed up the cache. But we never deployed that patch it 
on a big scale as it required resizing existing machines (more swap, no AFS 

The downside is of course that you loose your cache upon reboot. I guess 
disconnected-AFS users might consider that serious.

2. AFS files end up twice in memory, once in the mapping of the AFS file 
itself and then in the mapping of the cache chunk. We've addressed this by 
short-circuiting the VM layer for the AFS file, a relatively straightforward 
mod, but which gets messy as you still need that layer for everything that is 
memory-mapped, such as executables.

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