Re: [9fans] fdisk: reported number/size of cylinders changes

[Axel Belinfante <Axel.Belinfante@cs.utwente.nl>]

I booted plan 9 in the same way in both cases
(kernel from flop, fs over the net).

In /sys/src/libdisk/disk.c I found the following comment
near the opendisk code, which explains the difference between
opening a ``clean'' empty disk, and one already containing
a partition table. However, I would have expected that
reading (and using) the partition table on disk would
give me back the number previously shown (and, written,
I assume), by fdisk when I ran it first time.

Axel.

Comment found in /sys/src/libdisk/disk.c:

 * Discover the disk geometry by various sleazeful means.
 *
 * First, if there is a partition table in sector 0,
 * see if all the partitions have the same end head
 * and sector; if so, we'll assume that that's the
 * right count.
 *
 * If that fails, we'll try looking at the geometry that the ATA
 * driver supplied, if any, and translate that as a
 * BIOS might.
 *
 * If that too fails, which should only happen on a SCSI
 * disk with no currently defined partitions, we'll try
 * various common (h, s) pairs used by BIOSes when faking
 * the geometries.

[maybe I should check that for the clean disk the
 second case is used, and not the third]

> Did you happen to get to plan 9 differently in the two instances?
> It could be that you're just seeing two different logical geometries
> of the same disk.  In LBA mode, we don't use the logical geometries
> except to determine the number of disk blocks, so you should be OK
> in both cases (modulo the fact that one comes out to less sectors).