On Fri, Mar 11, 2011 at 02:26:56AM +0100, Markus Schatzl wrote: > Thanks Otto, > > this is somehow obvious, nevertheless there are tools relying on the > partition layout, like disklabel (at least regarding the start of the > partition). > > So essentially, every start above the first sector (ie. the MBR) would be > acceptable? I see that the installer suggests sector 2 as start of the > disk (in fdisk), however disklabel starts wd0a at offset 64 (default), > which seems kind of odd. Is there an explanation for this effect?
Here follows a lot of guesses that might put you on the right track. I have never had time to thoroughly investigate this... Grub likes to put its stage 1.5 (i think) bootloader in the sectors following the MBR. It can also avoid that, but how it deduces that these sectors are free to use I do not know. Somwhere on the disk the BSD disklabel has to bee. I guess it is after the PBR (that has to be the first sector of a primary partition since that is what the MBR loads and executes). For OpenBSD there is only one disklabel on the disk so I guess that is why there can be only one A6 (OpenBSD) primary partition on the disk. FreeBSD i think can slice/partition all 4 primary partitions with individual disklabels. If you use the whole disk for OpenBSD or FreeBSD it is quite possible the first sectors after the MBR is used for the disklabel. The number 64 should stem from the max number of sectors on one track in the C/H/S notation, which is 63, making sector 64 the first sector on the second track. So traditionally you leave the first track free to play with for bootloaders, disklabels and such. > > Amit: You're right. However, as disks from about 2005 onwards are larger > than this boundary, you always can _accidentally_ choose to make your boot > partition too big. As the C/H/S configuration is kind of an advanced > option, I'd believe in a healthy common sense of people using this option, > instead of preventing them to accomplish the task they're after. > > Thanks in advance, > /Markus -- / Raimo Niskanen, Erlang/OTP, Ericsson AB
