Re: Help with CHS / geometry

[Mouse <mouse%Rodents-Montreal.ORG@localhost>]

>> sd0: 8715 MB, 8387 cyl, 10 head, 212 sec, 512 bytes/sect x 17850000 sectors

> [...] 17780440 sectors total.

> But the drive has 17850000 sectors, as it does not really have that
> geometry physically (and nothing cares much about that fact).

Indeed.

Most of the reasons the OS once cared about geometry are no longer
valid, and, in these days of ZBR, even real spinning rust no longer has
a single geometry anyway.

Personally, I usually do one of two things: either (1) I factor the
sector count and pick C/H/S numbers that are in reasonable ranges and
multiply out to the correct value (for 17850000 sectors I'd likely pick
1700 cyl, 42 head, 250 sec), or (2) I use H and S values that give 1M
"cylinders", for disklabeling convenience.  On a few occasions I've
gone for 2M or 4M cylinders instead, for disks over 64G (for which the
count of 1M "cylinders" overflows a 16-bit field - all the fields of
struct sun_dkgeom are 16-bit...but, confusingly, sdkp_cyloffset is 32
bits).

> The last 17850000 - 17780440 = 69560 sectors will not be accessible
> with this "virtual" geometry and a sun label.  The kernel will print
> a note about this on boot, but this is fine (and it happens for a lot
> of "real" drives).

Also, don't be afraid to experiment with things such as partitions
placed such that the extend - or even begin - past what the geometry
information says is the end of the disk.

One more thing you can do is use a Sun label that describes only one
partition, in which you put your boot stuff, with a native NetBSD
label, including other partitions, elsewhere.

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