[OpenAFS] Re: Linux tmpfs
Douglas E. Engert
Wed, 19 Nov 2008 09:55:32 -0600
Rainer Toebbicke wrote:
>> 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
> Yes, it does, it just reads cache files like any vanilla user space
> 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.
It looks like it should be easy. It was for Solaris 10 so it can run
on tmpfs or ZFS. See http://rt.central.org/rt/Ticket/Display.html?id=123677
all the code change is in osi_file.c, and the param.<sysname>.h files.
> 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 cache).
> 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.
Douglas E. Engert <DEEngert@anl.gov>
Argonne National Laboratory
9700 South Cass Avenue
Argonne, Illinois 60439