GEOM meaning

[Frank Leonhardt]

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.