[4174] in arla-drinkers
Re: Arla on FreeBSD
daemon@ATHENA.MIT.EDU (Tomas Olsson)
Thu Feb 15 10:43:14 2007
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From: Tomas Olsson <tol@it.su.se>
To: Robert Watson <rwatson@FreeBSD.org>
Subject: Re: Arla on FreeBSD
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Date: 15 Feb 2007 16:42:48 +0100
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Cc: Kostik Belousov <kostikbel@gmail.com>, freebsd-fs@FreeBSD.org,
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Robert Watson <rwatson@FreeBSD.org> writes:
> On Thu, 15 Feb 2007, Tomas Olsson wrote:
> >> Also, you could look at file handle API, that would save you of path
> >> lookups after the vnode is looked up first time (look around for vfs_vptofh
> >> and vfs_fhtovp ops). This API is used by NFS server, so it shall work :).
> >>
> > That's an idea. We used that before in another context, but then we ran into
> > trouble with too many OSes so we dropped it. Depends on how much code one
> > can share.
>
> It has been ages since I looked at the xfs/nnpfs internals, but I thought it
> already used file handles to identify cache files to the kernel? I seem to
> recall several of our fh*() system calls were added by Assar specifically in
> support of Arla work.
>
Sounds likely. I first saw it from arlad's side, so I've always thought of
it as an optimization to avoid lookups in that end without pondering it all
that much.
> Is it the case that one file visible in /afs may now be backed by pages
> coming from different cache files?
>
Yup. There ware requests for low latency fetching of ID3 tags etc, and
working with files larger than the cache.
> How does this interact with the vnode pager? I assume you actually end
> up with two pagers -- one for the file as visible in /afs, and then
> satisfy the pager's VOP requests by forwarding them to the right one of
> the cache vnodes?
>
Exactly. As always, just more fine grained.
> I trust you don't mmap the cache vnode, which would lead to cache
> consistency issues between any mappings of the /afs vnode vs the cache
> vnode?
>
Me too :-)
But this is still a suboptimal world where we end up with double pager
layers, and consistency can still be a problem. In the long term it would
be nice to have arlad open it's cache nodes through nnpfs, be it small
files or just one large VM space.
> Any chance we could attract you to BSDCan to meet in person to discuss this?
>
Chance, not probable; May is beyond my horizon. And I do expect to have the
arla working properly on fbsd well before that.
/t
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