[Xml-bin] Some central design issues

Stephen D. Williams sdw@lig.net
Fri, 13 Apr 2001 13:55:43 -0400


Al Snell wrote:
> 
> On Fri, 13 Apr 2001, Stephen D. Williams wrote:
...
> > Your thinking of grouping like fixed sized data items together is good
> > and will improve efficiency, but that's really a 'compression' level
> > problem.
> 
> It also helps simplify free-space management - which is trivial for fixed
> sized lumps.

But you trade that for blowing, to some extent, locality of reference.

> > 'Elastic Memory' is simply the idea that within a block, you can have
> > holes that are hidden from higher layers but are used, and created, by
> > modification to the virtually flat and contiguous memory space.
> 
> I designed a filesystem once that supported this directly (when you do a
> write, you get to say whether the byte sequence should overwrite or insert
> - and you could delete bits too. Moving things within the file was done as
> an intrinsic operation rather than a read then write, in order to allow
> for optimisation of disk->disk transfers in the controllers), since so
> many files require it!
> 
> It also supported bookmarks - you drop a bookmark at a file position, and
> get a handle returned. A position in the file is specified as a bookmark
> handle plus or minus a number of bytes. Bookmarks move around when bytes
> are inserted or deleted.

Cool, similar.

> The filesystem worked by treating the disk as a single file, with the
> semantics described above, in the disk driver.
> 
> The filesystem basically mapped a lot of files down into one file, and
> could be recursively used if you so wished.
> 
> If you have efficient low level operations to model a byte string that can
> be chopped about, the rest gets a lot easier :-)

That's the goal of 'elastic memory' with vpointers.

> ABS
> 
> --
>                                Alaric B. Snell
>  http://www.alaric-snell.com/  http://RFC.net/  http://www.warhead.org.uk/
>    Any sufficiently advanced technology can be emulated in software
>

sdw
-- 
sdw@lig.net  http://sdw.st
Stephen D. Williams
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Dec2000