Date: Wed, 3 Dec 2003 00:29:14 +1030 From: Malcolm Kay <malcolm.kay@internode.on.net> To: Ion-Mihai Tetcu <itetcu@apropo.ro>, freebsd-questions@freebsd.org Subject: Re: fdisk question (long) Message-ID: <200312030029.14531.malcolm.kay@internode.on.net> In-Reply-To: <20031202125044.574ca489.itetcu@apropo.ro> References: <20031202125044.574ca489.itetcu@apropo.ro>
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On Tue, 2 Dec 2003 21:20, Ion-Mihai Tetcu wrote: > Hope someone will have the pacince to read all this ... > > I have a 120G HDD, in the BIOS is set as LBA. I've RTFM as much as I > could, but there still are some things I clearly don't understand. I > want to be sure that I can move this disk to another machine with > anouter BIOS and the system still boots up. > > I've used sysinstall to make partitions and the result is bellow: > > > it# fdisk ad0 > ******* Working on device /dev/ad0 ******* > parameters extracted from in-core disklabel are: > cylinders=3D232578 heads=3D16 sectors/track=3D63 (1008 blks/cyl) > > Figures below won't work with BIOS for partitions not in cyl 1 > parameters to be used for BIOS calculations are: > cylinders=3D232578 heads=3D16 sectors/track=3D63 (1008 blks/cyl) > These figures are just figures that will probably work -- and are unlikely to have any connection to the physical disk structure. > Media sector size is 512 > Warning: BIOS sector numbering starts with sector 1 > Information from DOS bootblock is: > The data for partition 1 is: > sysid 165 (0xa5),(FreeBSD/NetBSD/386BSD) > start 63, size 497952 (243 Meg), flag 0 > beg: cyl 0/ head 1/ sector 1; > end: cyl 30/ head 254/ sector 63 Having sthe geometry set to 23578/16/63 (that is sixteen heads) it is rather strange to address head number 254. Maximum head=20 number should then be 15. However it is also quite common to define large disk geometries as nnnnnn/255/63 which allows a maximum head number of 254. > The data for partition 2 is: > sysid 6 (0x06),(Primary 'big' DOS (>=3D 32MB)) > start 498015, size 41929650 (20473 Meg), flag 0 > beg: cyl 31/ head 0/ sector 1; > end: cyl 1023/ head 254/ sector 63 The CHS descriptor has overflowed -- nolonger meaningful. LBA works with the 498015/41929650 figures. > The data for partition 3 is: > sysid 165 (0xa5),(FreeBSD/NetBSD/386BSD) > start 42427665, size 192008880 (93754 Meg), flag 0 > beg: cyl 1023/ head 255/ sector 63; > end: cyl 1023/ head 254/ sector 63 > The data for partition 4 is: > <UNUSED> > > Q1: How can the partition 3 end up before beginning ? > The CHS entries are limited to 1023/255/63 which does not come anywhere near the disk capacity -- so once C reaches 1023 the CHS recording capaci= ty=20 has been exceeded. Or to put it another way the CHS entries are somewhat meaningless on large disks. But the absolute start sector number and slic= e=20 size is also recorded in the slice/partition table and this is used in LB= A=20 mode. > Q2: What is the Warning: BIOS sector numbering starts with sector 1 > trying to say ? The cylinders=3D232578 heads=3D16 sectors/track=3D63 sh= ows the > same as in teh BIOS screen. > Cylinder and head numbering starts at 0; sectors start at 1. A quirk of history that you need to know when using CHS. > Q3: The in-core parameters and those for BIOS calculation are the same; > this normal (from my experince) / when they won't mach ? > > So i decided to make it by hand (note that sysid 0 for the first > partition is a typo - it should be 165 and I'll want the / slice on it, > and I want to reserve the second partition for a winXP, and the 3rd wil= l > be for the other slices). > =2E.. > Q4: I've supplied the start and size parameters by reading those > provided by the sysinstall partitioning. How can I calculate them ? > Work with absolute sector numbers but chosen so that a slice always=20 starts at sector 1 in the CHS scheme. > Q5: Why the new parameters are different from those of sysinstall ? > Possibly a change of assumed CHS geometry > Q6: Is this schema OK and will I be able to use this disk in an other > computer and access all the partitions and slices ? Probably but I would feel happier with sysinstall generated values. Malcolm
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