Date: Tue, 12 Jun 2007 13:43:37 +0100 From: Rui Paulo <rpaulo@fnop.net> To: Matthew Dillon <dillon@apollo.backplane.com> Cc: freebsd-geom@freebsd.org, freebsd-hackers@freebsd.org, Chuck Swiger <cswiger@mac.com>, Marcel Moolenaar <xcllnt@mac.com>, freebsd-current@freebsd.org Subject: Re: GPT - (last) call for action Message-ID: <86ejkhcphi.wl%rpaulo@fnop.net> In-Reply-To: <200706112033.l5BKXgNf052683@apollo.backplane.com> References: <f4ek4o$i4u$1@sea.gmane.org> <4AB3C4C0-0DA1-482F-A4CD-375A53332F29@mac.com> <f4f14d$lhe$1@sea.gmane.org> <4D7CDA24-48FE-4319-A320-C8D7165E9EBC@mac.com> <200706092128.l59LSjRs027671@apollo.backplane.com> <57F8CCC1-1841-41AE-9F82-0C87FE53BE99@mac.com> <200706101752.l5AHqdE0035954@apollo.backplane.com> <8B01C1EC-D61A-484F-B308-6D6C8EB00EE6@mac.com> <861wgjwnrw.wl%rpaulo@fnop.net> <200706102143.l5ALhQut038340@apollo.backplane.com> <86zm37v4ns.wl%rpaulo@fnop.net> <B76664DB-2AC5-4196-979A-EF6E36FAFDCD@mac.com> <86lkeqxo89.wl%rpaulo@fnop.net> <EED5019B-C141-454A-8C5C-6F5E9A294FD8@mac.com> <200706112033.l5BKXgNf052683@apollo.backplane.com>
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At Mon, 11 Jun 2007 13:33:42 -0700 (PDT), Matthew Dillon wrote: > > > :>> # gpt -r show /dev/rdisk0 > :>> start size index contents > :>> 0 1 PMBR > :>> 1 1 Pri GPT header > :>> 2 32 Pri GPT table > :>> 34 6 > :>> 40 409600 1 GPT part - C12A7328-F81F-11D2-BA4B- > :>> xxxxxxxxxxxx > :>> 409640 159414704 2 GPT part - 48465300-0000-11AA-AA11- > :>> xxxxxxxxxxxx > :... > :> Well, what's happening is that Boot Camp syncs the BIOS partition > :> table with the GPT table, so the first partition should start at 40, > :> just like the GPT. > :> > :> Why does it start at 40 ? Because you need room for the PMBR, the > :> Primary GPT header and the Primary GPT table. > : > :Agreed, you need about 32 sectors for the GPT header+table. > > It makes sense for them to point the first MBR slice at the first > partition in the GPT, even though the standard says something else. > > It really sounds like they are making an accomodation for BIOS > booting or older Windows booting... or *something* of that sort. The > fact that the bootability bit is not set in the MBR (I'm not sure about > that, is it set or not?)... that seems to imply a compatibility issue > with other OS's like Windows in a multi-boot environment. > > They are just doing it all with a single slice instead of having > two slices. > > I'll bet they found that the two-slice method doesn't work in some > cases and the one-slice method does. The standard document doesn't > allow either method but it does seem to be a bit less insistent on > the starting sector for slice 1 then it does on there only being > one slice in the MBR, period. I can also see some OS's / disk managers > barfing on having two slices which overlap each other. > > So it really does make sense for them to point the MBR at sector 40. > The more I think about it, the more sense it makes. And also, if they used two partitions that would mean you would only have one partition left for installing Windows. -- Rui Paulo
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