Subject: bin/6624: NetBSD (in particular "disklabel") should probably not use sectors/track and sectors/cyl. on SCSI disks...
To: None <email@example.com>
From: None <firstname.lastname@example.org>
Date: 12/20/1998 23:14:29
>Synopsis: NetBSD (in particular "disklabel") should probably not use sectors/track and sectors/cyl. on SCSI disks...
>Responsible: bin-bug-people (Utility Bug People)
>Arrival-Date: Sun Dec 20 20:20:01 1998
>Originator: Greg A. Woods
Planix, Inc.; Toronto, Ontario; Canada
Modern disks, including almost any SCSI disk that still spins
today, use constant density recording techniques that make the
number of sectors per track.
As a result many modern (SCSI) disks also totally hide the true
geometry of the disk (usually behind a RAM buffer). The actual
sector sequence layout of the disk may be completely different
than any driver could ever guess without knowing the on-board
controller firmware intimately.
Operating system software that still calculates disk
partitioning and filesystem parameters based on recording
surface count, sectors per track, cylinders per spindle,
rotational speed, interleave, etc., is likely to be so
incredibly wrong that disks with small cache buffers will likely
be slower than if the OS simply ignored all of this faked up
geometry information (though this is something I've yet to see
proven). Even if it's not all that much slower, the additional
unnecessary complexity and confusion should be avoided for the
Obverve that on some modern disks as much as 1-2 MB can be lost
due to rounding errors:
sd2: 4345MB, 6144 cyl, 8 head, 181 sec, 512 bytes/sect x 8899737 sectors
8 heads * 181 avg.sect/trk = 1448 sect/cyl
1448 sect/cyl * 6144 cyl = 8896512 sectors
8899737 - 8896512 = 3225 lost sectors
That's only 0.03% of the available disk space, but still! ;-)
Make disklabel smarter so that it ignores sectors/track on SCSI
disks where such a measurement is meaningless (or "average" at
best). This might not be too difficult, assuming the user
manages to put the right keyword into the "type" field to hint
that the sectors/track value is an "average" value.
Disklabel should also ignore cylinder boundary checks except on
those architectures (sun3 + sparc) where it is known that the
PROM refuses to boot from partitions that don't "jive".
Make FFS ignore disk geometry on modern disks. This is not so
easy to do, unfortunately....