[Linux-cachefs] Re: CacheFS

[David Howells]

Erik Mol <kokkers at gmail.com> wrote:

> Still some of the CacheFS parts are still a bit vague to me.
> FScache!=CacheFS!=Cachefiles(d)

Have you looked at my OLS presentation?  It's at:

	http://people.redhat.com/~dhowells/fscache/fscache-ols2006.odp

> FS-Cache is a part of the Linux Kernel which intermediates between  'network
> based filesystems' (AFS/NFS) and a cache back-end.
> FS-Cache is designed to work with a cache back-end of choice.

With one or more back-ends of choice, yes.  You aren't limited to one.

> Cache back-ends:
> CacheFS can act as a cache back-end to FS-Cache and runs on top of a  block
> device thus providing its own filesystem.
> CacheFiles can act as a cache back-end to FS-Cache and runs in  directory on
> top of another filesystem.

Both correct.

> We are using and talking about CacheFiles(d), right?

Probably.  CacheFiles is the cache back-end, comprising a module and a
userspace program (called cachefilesd) that manages the cache.

> Since I do not have the knowledge to modify a RHEL4 distribution to  get it to
> work with FS-Cache I decided to try out RHEL 5 Beta 2.

You'd have to replace the kernel in RHEL4 with something much more recent.

> Dec  6 11:59:37 RHEL5B2 cachefilesd[2117]: Failed to check object's  in-use
> state: errno 95 (Operation not supported)

That very much suggests that cachefilesd was unable to access user xattrs on a
file.

You could try turning up the debugging level on cachefilesd (-d), telling it
to log to stderr (-s) and running it in command line mode (-n) and then
capturing the output:

	cachefilesd -dsn | tee cache.log

to see what it's doing.

David