From owner-freebsd-fs  Thu Aug 19 20:11:20 1999
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Date: Thu, 19 Aug 1999 20:10:57 -0700
From: Chuck Silvers <chuq@chuq.com>
To: Terry Lambert <tlambert@primenet.com>
Cc: wrstuden@nas.nasa.gov, Hackers@FreeBSD.ORG, fs@FreeBSD.ORG
Subject: Re: BSD XFS Port & BSD VFS Rewrite
Message-ID: <19990819201057.A2185@chuq.chuq.com>
References: <Pine.SOL.3.96.990818112953.14430G-100000@marcy.nas.nasa.gov> <199908182043.NAA28863@usr06.primenet.com>
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In-Reply-To: <199908182043.NAA28863@usr06.primenet.com>; from Terry Lambert on Wed, Aug 18, 1999 at 08:43:14PM +0000
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On Wed, Aug 18, 1999 at 08:43:14PM +0000, Terry Lambert wrote:
> > > > Nope. The problem is that while stacking (null, umap, and overlay fs's)
> > > > work, we don't have the coherency issues worked out so that upper layers
> > > > can cache data. i.e. so that the lower fs knows it has to ask the uper
> > > > layers to give pages back. :-) But multiple ls -lR's work fine. :-)
> > > 
> > > With UVM in NetBSD, this is (supposedly) not an issue.
> > 
> > UBC. UVM is a new memory manager. UBC unifies the buffer cache with the VM
> > system.
> 
> I was under the impression that th "U" in "UVM" was for "Unified".
> 
> Does NetBSD not have a unified VM and buffer cache?  is th "U" in
> "UVM" referring not to buffer cache unification, but to platform
> unification?
> 
> It was my understanding from John Dyson, who had to work on NetBSD
> for NCI, that the new NetBSD stuff actually unified the VM and the
> buffer cache.
> 
> If this isn't the case, then, yes, you will need to lock all the way
> up and down, and eat the copy overhead for the concurrency for the
> intermediate vnodes.  8-(.

netbsd w/UVM currently doesn't have unified caches.  that feature is
what I named UBC, for "unified buffer cache" (ala DEC's UBC).
the U in UVM doesn't actually stand for anything.  :-)


> > > You could actually think of it this way, as well: only FS's that
> > > contain vnodes that provide backing should implement VOP_GETPAGES
> > > and VOP_PUTPAGES, and all I/O should be done through paging.
> > 
> > Right. That's part of UBC. :-)
> 
> Yep.  Again, if NetBSD doesn't have this, it's really important
> that it obtain it.  8-(.

I'm workin' on it... it'll go in soon after the branch for the next release
is created (ie. it won't be in the next release, but the one after that).

-Chuck


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