Date: Mon, 23 May 2011 08:19:16 -0600 From: Warner Losh <imp@bsdimp.com> To: "Andrey V. Elsukov" <ae@freebsd.org> Cc: Adrian Chadd <adrian@freebsd.org>, src-committers@freebsd.org, svn-src-all@freebsd.org, Marcel Moolenaar <marcel@xcllnt.net>, Stefan Farfeleder <stefanf@freebsd.org>, svn-src-head@freebsd.org Subject: Re: svn commit: r221972 - head/sys/geom/part Message-ID: <DA1D5895-4959-49F4-9621-18F2CF1D809B@bsdimp.com> In-Reply-To: <4DD9F0A3.701@FreeBSD.org> References: <201105152003.p4FK3tnS050889@svn.freebsd.org> <20110522093302.GA2638@mole.fafoe.narf.at> <BANLkTikoBK4ZCHB488eRgbySPBcXC0nnow@mail.gmail.com> <BD2C1F30-854A-4AF2-A9D8-654F3E4E84A8@bsdimp.com> <6AE10D76-AC2F-4D7B-A985-EE072949ECC4@xcllnt.net> <42C49AE5-C8EA-44A0-AF88-16130BACE912@bsdimp.com> <4DD9F0A3.701@FreeBSD.org>
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On May 22, 2011, at 11:29 PM, Andrey V. Elsukov wrote: > On 23.05.2011 6:29, Warner Losh wrote: >> Looking at one of my flash drives that shows the problem: >>=20 >> da0: 15423MB (31588351 512 byte sectors: 255H 63S/T 1966C) GEOM_PART: = partition 1 has end offset >> beyond last LBA: 31588350 > 31588325 >>=20 >> So why does gpart think the last LBA is 25 less than it really is. = Well, let's do some sanity >> checks first. fdisk -s da0 tells us: >=20 >> I'm pretty sure this problem is due to a bug in g_part_mbr.c: >>=20 >> basetable->gpt_last =3D msize - (msize % basetable->gpt_sectors) - 1; >>=20 >> This is wrong, or at least it is a widely disregarded part of what = makes up an MBR. When I >> correct the size, the geom code is fine. There's no requirement in = MBR that a partition end of >> any particular boundary, although sometimes you'll find mistaken = documentation that suggests this >> is the case. Reading between the lines at = http://www.boot-us.com/gloss03.htm suggests that this >> restriction was only for disks < 8GB in size (from the fact it said = that all the partitions can >> be described with the CHS fields). This is one of the things I = learned when I tried to make >> fdisk enforce that: this isn't a requirement of MBR as it is = implemented in the wild today. >=20 > There is another opinion: > http://www.boot-us.com/gloss02.htm >=20 > "There is the convention that partitions should always start and end = on *cylinder boundaries*." The current code doesn't do that. The current code just ends on a track = boundary... > but >=20 > "Linux and Windows NT/2000/XP/2003/Vista/2008/7 seem to have no = problems when partitions do not > start or end on cylinder boundaries. However other operating systems = (e.g. DOS) expect that this > convention is obeyed." >=20 > What way will we choose? We choose the way that works and not bother with alignment. It is an = artifact of the < 8GB disks and hasn't really been necessary for about = 15 years now. > I prefer to: leave the choice to the user how partition should be = aligned, and remove automatic > alignment to track boundaries from MBR (and probably from all other = schemes). I think we agree. Allow, but do not force, alignment. Warner > --=20 > WBR, Andrey V. Elsukov >=20 > --=20 > WBR, Andrey V. Elsukov >=20
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