UEFI partition - dual boot

Juszr · 2013-07-31 16:48:58

I have installed Win 8 on my laptop, so i have an EFI partition already (using GPT).. I read the guides, google but i didn't understand yet if i have to create another EFI partition or i could use the one created by windows (and if so how do i share it for both arch and win 8)?

- Something else: It says in the guide that it has to be min. 512MB, mine is 100MB (because that's what windows has created), is that correct?

Btw already installed rEFInd.

Last edited by Juszr (2013-07-31 18:02:43)

srs5694 · 2013-07-31 19:37:12

The EFI System Partition (ESP) is meant to be shared between OSes. As per the EFI spec, it's legal to create multiple ESPs on a disk, so you can do it that way; but the Windows 7 installer flakes out when it sees multiple ESPs, so it's not really advisable. (I don't know offhand if Windows 8 is better in this respect.)

As to the ESP's size, the 512MiB (note that's MiB, not MB; 512MB is smaller than 512MiB) size is based on the fact that some users have had problems with sub-512MiB FAT32 ESPs; the EFI can sometimes ignore files or otherwise misbehave on sub-512MiB FAT32 ESPs. The solutions are to increase the size of the filesystem or to convert to FAT16. Using a FAT16 ESP is technically illegal, and Windows also tends to flake out with it, but most EFIs work fine with it. In the end, therefore, a FAT16 ESP is a good solution only on a Linux-only system. As a practical matter, you should probably leave it as-is and hope for the best, since resizing the ESP is likely to be a pain. If you run into weird problems, though, you may need to bite the bullet and either resize the partition or create another ESP (and perhaps copy the first one's files to it and then retire it, if Windows compatibility becomes an issue). On new installations in the future, create a 550MiB ESP before installing any OS. (550MiB is big enough that mkdosfs creates FAT32 on it by default, and is big enough to work even if there's a MB/MiB mixup.)

mrunion · 2013-08-01 01:36:04

FWIW, my new laptop came with Windows 8 pre-installed. I created a NEW ESP and installed Arch and Win8 hasn't complained. I do not know if Win7 will, but Win8 has NOT had an issue so far.

srs5694 · 2013-08-01 13:32:53

mrunion wrote:

FWIW, my new laptop came with Windows 8 pre-installed. I created a NEW ESP and installed Arch and Win8 hasn't complained. I do not know if Win7 will, but Win8 has NOT had an issue so far.

The problem I've experienced is with the installer. There may also be issues in Windows itself when accessing the ESP (say, when using the "bcdedit" command), but I haven't tested them. Such problems would be unlikely to crop up in day-to-day operations.

The issue here is that creating a second ESP is like setting a time bomb for your computer. Windows has a tendency to require re-installation from time to time, and if you've got two ESPs, that re-installation might well fail. There are workarounds, of course -- you could use gdisk to change the type code of the Linux ESP to something else, then change it back (if necessary) to get Linux to boot. Personally, though, I prefer setting things up so that you don't need such workarounds in the future.

Of course, all this is predicated upon the assumption that the Windows 8 installer is as flaky as the Windows 7 installer. So far I've managed to avoid Windows 8, so I can't comment on Windows 8 in this respect. Given Microsoft's history, though, it's entirely possible that Windows 8 has the same problem as Windows 7.

Juszr · 2013-08-01 17:02:49

Ok, thank you for answering.

To use the ESP do i have to format the partition (and then add refind, or gummiboot...)?

WonderWoofy · 2013-08-01 17:28:48

If you are still speaking of the ESP that is already on your system (presumably the one created/used by windows), then no, don't format it... unless you don't want windows to boot, which might not be an unreasonable desire.

Just use the ESP as is. Mount it at /boot/efi (or /boot is thats how you want to go about it). If you recreate the filesystem, then you are going to wipe out the efi applications that are used to boot windows. You just need to add the Linux files there, not make them exclusive.

TheSaint · 2013-08-02 07:57:02

If I'm correct, only one ESP has boot flag set, which would make BIOS to chose it.

srs5694 · 2013-08-02 15:27:57

TheSaint wrote:

If I'm correct, only one ESP has boot flag set, which would make BIOS to chose it.

In reality, there is no such thing as a "boot flag" on a GPT disk. That said, the libparted library, and therefore tools based on it (such as parted and GParted) use the "boot flag" to identify partitions with a type code of C12A7328-F81F-11D2-BA4B-00A0C93EC93B -- that is, an ESP. Thus, in libparted terms, you can set the "boot flag" on as many partitions as you like, although you should do so only on FAT partitions that hold EFI boot loaders. Also, as noted earlier, the Windows installer tends to flake out when multiple ESPs exist on the disk. The EFI's NVRAM entries determine the order in which files are booted, and these files can reside on any ESP, or on a mixture of them. This last point is very important: In the EFI world, the boot process operates on files; you don't "boot a partition" in the way you do with BIOS.

In the BIOS/MBR world the "boot flag" is real and determines the partition that Microsoft's first-stage boot loader will boot. Unfortunately, the libparted developers decided to re-use the "boot flag" terminology in GPT, applying it to a concept (the ESP type code) that's only very vaguely related to its meaning on MBR. This poor choice has caused a great deal of confusion.

Arch Linux

#1 2013-07-31 16:48:58

UEFI partition - dual boot

#2 2013-07-31 19:37:12

Re: UEFI partition - dual boot

#3 2013-08-01 01:36:04

Re: UEFI partition - dual boot

#4 2013-08-01 13:32:53

Re: UEFI partition - dual boot

#5 2013-08-01 17:02:49

Re: UEFI partition - dual boot

#6 2013-08-01 17:28:48

Re: UEFI partition - dual boot

#7 2013-08-02 07:57:02

Re: UEFI partition - dual boot

#8 2013-08-02 15:27:57

Re: UEFI partition - dual boot

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