[OpenAFS-devel] Progress on Linux in-kernel RxRPC library

Tue, 22 Mar 2005 00:28:50 -0500

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--On Monday, March 21, 2005 08:01:06 PM -0500 Kyle Moffett=20
<mrmacman_g4@mac.com> wrote:

> So do you use the PAG to reference a per-user index of access
> control rights for a collection of files, or do you use the PAG to
> check access against the cached copy in a global shared cache?
I can't tell what you are asking here, so I'll describe the datastructure=20
that is actually used.
Each cached vnode has a pointer to a linked list of struct axscache=20
entries. Each entry maps a pag/user id to it's effective access rights.

>  In the latter case,
> you would just cache the ACL sent by the fileserver and check
> against the principal name/id stored in the key and global cache.
The client does not compute the access rights by evaluating the ACL. The=20
client gets the access rights from the fileserver. The fileserver computes=20
the _effective_ rights of the caller on every FetchStatus request and=20
returns that information to the client. The client updates the axscache=20
entry as part of processing the FetchStatus response. later permission()=20
calls (and some internal users) can use this information instead of asking=20
the fileserver, as long as the callback is still valid..

> I'm just trying to understand what part of the existing Linux
> filesystem cache is insufficient for OpenAFS to utilize without
> resorting to patched-in external caches.
Not that this has anything to do with PAGs, what rx implementation we use,=20
or the axscache...

Are you talking about the current pagecache, or the as yet non-mainline=20
cachefs?

If the former:
The afs data cache's working set can be larger than the memory reserved for =

the pagecache.
The afs data cache persists across reboots. The client validates stored=20
chunks by comparing the dataversion stored in the cache with that reported=20
by the fileserver. if the dv is the same, the cached data is valid and may=20
be returned to clients without transferring them over the network.

If the latter, then it probably is or could be made sufficient. The=20
following things would be needed.
- The ability to choose the 'key' that identifies cached objects. afs will=20
always need to index things by (cell,volume,vnode,uniquifier), and not by=20
filename. If cachefs doesn't help with chunking, tjhen the chunk number or=20
offset would also need to be part of the key.
- the ability to store metadata about each cached object (the DV and some=20
flags)
- A place to persistently store global metadata (the cell name to id map)

I think the following would also be needed:
- The ability to do range searches (partial key matches) (so that=20
everything in a (cell, volume) could be evicted from the cache on an fs=20
flushv, or all chunks of a single file, if chunking is implemented by=20
having multiple cachefs objects)

It comes back to that portability issue though. Having a 'better' interface =

to use on linux does not help us on the other platforms, and actually=20
increases complexity by requiring some sort of abstraction layer that can=20
be used to provide uniform access to the two cache implementations. Also,=20
at what point do we cross the line alluded to in=20
<http://www.linuxmafia.com/faq/Kernel/proprietary-kernel-modules.html>, and =

become code "written for linux" instead of "(a filesystem) ported from=20
other operating systems"?
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