[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