GEOM meaning

[John Baldwin]

On Tuesday, February 04, 2014 7:03:17 am Frank Leonhardt wrote:
> On 04/02/2014 11:57, Frank Leonhardt wrote:
> > On 04/02/2014 10:46, Dieter Lange wrote:
> >> Hi,
> >>
> >> having looked at quite a few sites now, not just 
> >> <http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/geom.html> 
> >> I still cannot find out what "GEOM" means (wrt disks etc., not 
> >> geography or so).  It probably does not mean "Modular Disk 
> >> Transformation Framework". I am not talking of its use and/or 
> >> definitions, just the meaning of the abbreviation or word...
> >>
> >> Thanks+kind regards from the only person on the WWW who doesn't know...
> >> DL
> >
> > I've always assumed it was short for (disk) geometry - i.e. converting 
> > logical requests to match the disk geometry. Eh? Well, back in my 
> > youth we did talk about the "geometr"y of DASD (disk!). For example, 
> > how many platters (heads), cylinders (tracks) and sectors/track were 
> > present. With ATA and SCSI this has become less relevant as you only 
> > get to see the logical structure of a disk (a load of blocks 
> > sequentially numbered 0...n). You may well ask why anyone would call 
> > these parameters "geometry", but I can't think of any other better 
> > name for it, nor any other word in common use for referring to them 
> > (other than CHT). But a disk's geometry was highly relevant because 
> > you (the programmer) would either be responsible for moving the head 
> > (via a stepper motor) to the correct track, or at the very least, you 
> > had to be sensitive to where the head was on the disk when optimising 
> > your code.
> >
> > I've no proof whatsoever that this is why the geom library is so 
> > called - it could all be a complete coincidence. I don't remember 
> > hearing about "geom" on System V, nor on BSD until recently (late 1990s).
> >
> > Regards, Frank.
> >
> P.S. The reason why I'm not 100% happy with the above theory is that the 
> geom library sits between DEVFS and the device driver (pretending to be 
> a device driver to DEVFS). This doesn't seem to me the logical place for 
> geometry translations, but it wouldn't be the first time a name has 
> ended up migrating to another purpose.

I have always assumed it was short for geometry.  One of the original
uses cases for GEOM was handling partition tables, and MBR in particular
is (sadly) quite tied to disk geometry since it still specifies boundaries
both as LBAs and as C/H/S tuples.

-- 
John Baldwin