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  #31  
Old 19th March 2012, 12:50 PM
Gareth Jones Offline
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Re: Issuses in dual boot - Fedora 16 with Windows 7

Quote:
Originally Posted by chrismurphy View Post
The primary and backup GPTs are left intact. I will argue this is sufficient for anaconda/parted to ignore the now stale GPT, but they don't.
I think the problem there is that some people use 'hybrid' partitioning schemes, with both a GPT and an MBR table that matches for the BIOS-visible partitions (rather than merely a single 'protective' MBR primary partition). I'm not sure who does this, maybe MacOS/Windows dual-booters, but it's noted as an option in the gdisk manual. This means that choosing which table to honour when both are present (as they always are with GPT, even though the MBR is usually just a single partition) is not so straightforward, and without actually comparing the tables, the easiest solution is to assume that the GPT is accurate.
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  #32  
Old 19th March 2012, 03:59 PM
DBelton Offline
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Re: Issuses in dual boot - Fedora 16 with Windows 7

Well, it would be kinda hard to just arbitrarily assume one or the other is the correct one without actually scanning the drive and comparing it back to the GPT/MBR, and that would take quite a lot of time to do.

I'm not so sure that anaconda should just error out and not continue if it encounters this situation, though. Possibly it could give an option to select which is the correct one, and it could fix the drive based on the option? It could put up the information it sees in the MBR, and the GPT, ask which one is correct, and also give an option to cancel the install, doing nothing to the drive.

Just a thought, anyway..

And I ran into it here when I had a GPT formatted drive and was going to install Windows 7. I actually caused it myself by using dd to wipe the GPT instead of going through the correct way and using gdisk or something like that to do it for me. I had completely forgotten that it kept a backup GPT, which I wiped after it erred out on me (I used dd to wipe it as well )
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  #33  
Old 19th March 2012, 04:11 PM
Gareth Jones Offline
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Re: Issuses in dual boot - Fedora 16 with Windows 7

Yes, dd should be able to kill old GPTs, but not knowing which GPT was catching Anaconda out, whether Windows uses the MBR–part1 gap, and whether the last partition had data in its last sector, I didn't want to encourage that unless there was no better way!

Short of giving Anaconda a proper advanced mode for volume management (which it really needs anyway for dealing with btrfs RAID and sub-volumes...), I think the best solution is that suggested last time round, of defaulting to MBR for disks less than 2 TiB even if they're blank.

Last edited by Gareth Jones; 19th March 2012 at 04:21 PM. Reason: Off by a factor 1024...
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  #34  
Old 19th March 2012, 04:19 PM
DBelton Offline
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Re: Issuses in dual boot - Fedora 16 with Windows 7

I wasn't encouraging the use of dd.. I was saying that my use of dd was actually the reason I ran into the problem

Hopefully people see that and will decide not to use dd on their own drives, unless they also remember that there are 2 copies of the GPT.

(also in my case, it was a spare drive I had sitting on the shelf with no good data on it. I had it formatted GPT, and just did a quick dd on the first part of the drive, since Windows won't install to a GPT drive unless you are booting a uEFI machine.)
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  #35  
Old 19th March 2012, 07:08 PM
chrismurphy Offline
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Re: Issuses in dual boot - Fedora 16 with Windows 7

If you read the UEFI spec on what is a valid legacy MBR, and what constitutes a valid GPT, there is in no possible way to interpret a disk which lacks an 0xEE entry in the MBR, as GPT.

From UEFI 2.3.1 Errata A.
5.3.2 says:If a GPT formatted disk is reformatted to the legacy MBR format by legacy software, the last logical block might not be overwritten and might still contain a stale GPT. If GPT-cognizant software then accesses the disk and honors the stale GPT, it will misinterpret the contents of the disk. Software may detect this scenario if the legacy MBR contains valid partitions rather than a protective MBR (see Section 5.2.1).
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  #36  
Old 19th March 2012, 07:21 PM
Gareth Jones Offline
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Re: Issuses in dual boot - Fedora 16 with Windows 7

That doesn't adequately handle 'hybrid' tables, but given that they're non-standard and a bit of gdisk-hackery before and after install can get around the problem, it's probably okay for them not to be supported by the installer.
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  #37  
Old 19th March 2012, 08:03 PM
chrismurphy Offline
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Re: Issuses in dual boot - Fedora 16 with Windows 7

5.2.1 says a valid legacy MBR may be located at LBA 0 if GPT is not being used.
5.2.3 says a protective MBR may be located at LBA 0 if GPT is being used.

So it's reasonable that GPT data would only be honored when:
a.) The GPT is valid, passing the test in 5.3.2; and also
b.) The MBR is missing, invalid, or protective.

If it's a valid legacy MBR without an 0xEE entry, you're in a minefield and if you get blown up...well that's unfortunate as well as not intentional. Any ambiguity is specious. It's entirely wishful thinking, imaginary, and non-standard.

I'll have to try Boot Camp again, because my recollection is that the first entry is 0xEE and the 2nd entry is for the Windows volume it creates. But now I'm wondering, so what is the Windows installer looking for in the MBR? The lack of an 0xEE entry, or at least one non-0xEE entry?

---------- Post added at 11:32 AM ---------- Previous post was at 11:27 AM ----------

Quote:
Originally Posted by Gareth Jones View Post
That doesn't adequately handle 'hybrid' tables, but given that they're non-standard and a bit of gdisk-hackery before and after install can get around the problem, it's probably okay for them not to be supported by the installer.
Insofar as hybrid MBRs are insane, and Apple is crazy to force us to use them to dual-boot their hardware, there's in a sense two kinds of hybrid MBRs:

a.) The kind that have an 0xEE entry.
b.) The kind that lack any 0xEE entry.

The first kind could possibly be handled somewhat sanely, sorta like walking on a 6" wide ledge of marbles, with 100' drop off the edge.

The second kind cannot be distinguished, without human intervention, from a "valid legacy MBR" at all. This is perilous hackery. You're on the same ledge with 10 feet of land mines in front, behind, above, and below. You *might* be able to stay where you are.

---------- Post added at 12:03 PM ---------- Previous post was at 11:32 AM ----------

For what it's worth, I can't figure out how to use gdisk to create a hybrid MBR without an 0xEE entry in the MBR.
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  #38  
Old 19th March 2012, 08:52 PM
Gareth Jones Offline
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Re: Issuses in dual boot - Fedora 16 with Windows 7

Quote:
Originally Posted by chrismurphy View Post
For what it's worth, I can't figure out how to use gdisk to create a hybrid MBR without an 0xEE entry in the MBR.
Me neither.

---------- Post added at 06:52 PM ---------- Previous post was at 06:31 PM ----------

I think the 0xEE partition is needed just to protect the GPTs, so then you're right that if it's not there, the GPT should be ignored.
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  #39  
Old 19th March 2012, 09:14 PM
chrismurphy Offline
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macoschrome
Re: Issuses in dual boot - Fedora 16 with Windows 7

Right exactly.

For some reason Boot Camp 3.04 crashes in my Snow Leopard VM so I can't see what it does. But my distinct recollection is that it places the GPT and EFI partitions into the 0xEE entry in the MBR. And puts separate MBR entries for HFS+ and NTFS, as 0xAF and 0x07. So three MBR entries.

When I do this manually with gdisk, I stuff everything into the 0xEE partition, and expose only the last GPT partition (usually LVM or btrfs root) as the 2nd MBR entry, and mark it bootable. And Apple hardware is happy with this arrangement.
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  #40  
Old 21st March 2012, 03:57 AM
chrismurphy Offline
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macoschrome
Re: Issuses in dual boot - Fedora 16 with Windows 7

FYI.
https://bugzilla.redhat.com/show_bug.cgi?id=805272
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