Re: [IDEA] - run-length compaction of block numbers

[Valdis . Kletnieks]

On Mon, 19 Jan 2004 13:45:29 +0300, Hans Reiser said:

> >Does the extent-based disk allocation used by OS/360 in 1964 count? :)
> >  
> >
> Probably.  Tell me more about it.

Well, basically, a dataset was described by an entry in the Volume Table of
Contents (VTOC) with a something conceptually similar to an inode.  The actual
data area was described with a initial allocation, and up to 15 additional
"extents" (yes, hard limit of 15 extents per volume, although a dataset could
span volumes).  Each extent was described with start/end cylinder/track pairs.
You could allocate space by absolute tracks, by tracks, or by cylinders (of
course, space actually allocated was dependent on the device).  So you could
code in the JCL something like SPACE=(CYL,(10.5)) which would allocate 10
cylinders and up to 15 additional 5-cylinder spaces.  And yes, it was quite
possible to get hosed on fragementation.

The first 3 allocations were contained in the 'format 1' DSCB (Data Set
Control Block), and additional extents were in a 'format 3' DSCB.  Free
space was described in extents in a series of chained 'format 4' DSCB's.
(A single DSCB was only 140 bytes, so there's a lot of chaining;).

I don't know how IBM did disk space management on the earlier systems
such as the 1401, 7040, and 7090 series, but I'd suspect it was a similar
extent-based method.

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