Subject: Disklabel and non 512-byte media
To: None <email@example.com>
From: Steve Woodford <firstname.lastname@example.org>
Date: 10/13/2006 23:05:44
I've noticed an interesting oddity with disklabels on media with
physical sector size != DEV_BSIZE. Here's one I prepared earlier based
on a Samsung YP-U2 USB MP3 player:
sd0 at scsibus0 target 0 lun 0: <Samsung, YP-U2, 0100> disk removable
sd0: fabricating a geometry
sd0: 999 MB, 249 cyl, 64 head, 32 sec, 2048 bytes/sect x 511616 sectors
# disklabel sd0
# size offset fstype [fsize bsize cpg/sgs]
d: 2046464 0 unused 0 0 # (Cyl. 0 - 999+)
e: 511560 56 MSDOS # (Cyl. 0*- 249*)
Note that the size and offset fields for partition 'e' are specified in
terms of the physical sector size, namely 2048 bytes. Yet the raw
partition is specified in 512 byte sectors.
The reason for this is the 'e' partition's vital statistics are copied
straight from the MBR, whereas the raw partition's details are set in
sdgetdefaultlabel() after conversion to the DEV_BSIZE equivalent.
Right now I have MSDOSFS working on 2048 bytes/sector media based on the
patch in kern/17398, with an additional tweak to
bounds_check_with_label() to account for the disklabel discrepancy.
The alternative is to ensure disklabels are always specified in
DEV_BSIZE units (at least until we ditch fixed DEV_BSIZE).
I prefer the former aproach but I'd be interested in hearing others'