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Re: random access delta streams

From: Greg Stein <gstein_at_lyra.org>
Date: 2000-11-14 19:48:01 CET

On Tue, Nov 14, 2000 at 07:05:21PM +0100, Branko Cibej wrote:
> Jim Blandy wrote:
> >> Another question: when do you need random access to old versions?
> >> And if you do, wouldn't it make more sense to store them in the
> >> clear, anyway, without having to apply a series of deltas every
> >> time? That's what any caching scheme will boil down to, anyway.
> >
> > Greg S. asked some mild questions, and I'm inferring from there.
> > The thought arose from the intersection of these issues:
> > - Subversion wants to use deltas to store multiple versions of files
> > in a compact way.
> > - Subversion wants to provide a uniform interface for accessing both
> > old and new versions of nodes.
> > - HTTP or DAV has some way to ask about substrings of files.
> Oooh, that's right: HTTP/1.1 let's you pick a file to pieces.

Concretely: the Adobe Acrobat (PDF) Reader plugin will use ranges when it
detects a slow line. It will grab everything for the first page, then go
back and start fetching the rest of the document. [fast lines just suck in
the whole doc]

The subrange request also happens to apply to PUT, but that would probably
apply to a "working resource" (no delta, no compression). And/or we can
disallow subrange PUT in mod_dav_svn. I'm fine with deferring this one for a
bit (although the DAV filesystem in MacOS X might do subrange PUT, but it
won't really work against SVN since we don't have DeltaV "auto-versioning").

Regarding large files: the Oracle folks are doing a backend for mod_dav 1.0
specifically so they can store large media files. The work is coming from
their Intermedia group, and they're looking at multi-megabyte (even
gigabyte) documents. (and yes, they also plan to do versioning with those
buggers) With documents of that size, subrange fetching is very important.

> > I have no compelling application in mind; I'm just trying to meet all
> > the promises we're making.
> Well, personally I think it would be a great feature to have, and I've a
> hunch that it could be a huge performance gain in some situations. I've
> no feeling about /when/ we'll need it. I suggest you add this to IDEAS,
> and we can pull it out of there when the time's right..

Acrobat Reader. Today. I'd suggest the time is right :-)

[ interesting note: we've been using the Reader as a test case for Apache
  2.0's updated byterange handling mechanisms ]


Greg Stein, http://www.lyra.org/
Received on Sat Oct 21 14:36:14 2006

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