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Kaboosh
Apprentice
Joined: 10 Jun 2004
Posts: 162
Location: Edmonton, AB - Canada
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 Posted: Fri Sep 24, 2010 7:52 pm Post subject: Booting from LVM |
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I've been reading about partition limits, MBR-style, and GPT-style partitions and I sorta like what this guy has to say: http://insan-it.blogspot.com/2010/09/gpt-versus-lvm.html
That being said I've used LVM enough to be comfortable but I've never had an "all-LVM" volume without a partition table. First:
1) Is this possible? Can I boot off an LVM volume?
2) If not, than can I try this guy's idea and create a small (512MB) /boot partition and embed LVM into the remainder of the disk? Can you create an LVM physical volume at an arbitrary offset (in this case at the end of the partitions)? I've looked into using loopback to achieve this since losetup takes an offset but what is the size limitation on loopback devices?
3) Someone mentioned that GRUB2 can boot from LVM. I'll RTM and try to set this up...then I'll post back with results. 
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"The philosophy of one century is the common sense of the next." - Henry Ward Beecher |
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chithanh
Developer
Joined: 05 Aug 2006
Posts: 2158
Location: Berlin, Germany
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| Posted: Fri Sep 24, 2010 9:29 pm Post subject: |
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GRUB 2 can boot from LVM, by placing the necessary boot code in the reserved area between the MBR and the start of the first partition. But you still have to have one partition on the disk, containing your pv.
BTW that blog post is full of uninformed opinions. |
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frostschutz
Advocate
Joined: 22 Feb 2005
Posts: 2977
Location: Germany
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| Posted: Fri Sep 24, 2010 11:29 pm Post subject: |
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I boot off USB (regular grub and boot partition on the usb key) so yeah sure I could make the internal disk full LVM without any partitions whatsoever...
...but to what purpose?! I still partitioned the disk with GPT and put several LVM PVs onto GPT partitions of that disk. So should I ever need part of the disk as non-LVM (for example a Windows data partition for a dual boot system), I can free one of the PVs and format it with something else.
As far as I'm able to tell there is no speed penalty involved whatsoever with using LVM on partitions as opposed to direct on disk. Exception being the LVM tools themselves, creating new LVs takes a wee bit more time with several PVs as opposed to only one PV. However I don't go creating LVs all day.
| Quote: | | create a small (512MB) /boot partition and embed LVM into the remainder of the disk? |
That's usually how it's done. You don't even need a bootloader with LVM support, then. Normal good old Grub suffices, as it can load the kernel from the boot partition manually. The kernel itself, naturally, will have to support LVM, and come with an initramfs that initialized LVM, so it can find and mount the LVM root partition. genkernel does that for you, alternatively check the initramfs guide in the gentoo wiki |
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Kaboosh
Apprentice
Joined: 10 Jun 2004
Posts: 162
Location: Edmonton, AB - Canada
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| Posted: Sat Sep 25, 2010 4:18 am Post subject: No Go |
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OK...so if you create an LVM volume on a disk you can "reserve" a small amount of space by moving the initial label up to the 3rd sector:
| Code: | | pvcreate --labelsector 3 /dev/sda |
I did a hexdump and got:
| Code: |
00000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
00000600 4c 41 42 45 4c 4f 4e 45 03 00 00 00 00 00 00 00 |LABELONE........|
00000610 68 6b 72 e7 20 00 00 00 4c 56 4d 32 20 30 30 31 |hkr. ...LVM2 001|
00000620 63 6c 44 51 6a 32 44 58 4b 53 36 38 47 6b 6e 61 |clDQj2DXKS68Gkna|
00000630 32 50 36 76 6a 44 32 49 59 6f 74 59 39 78 36 67 |2P6vjD2IYotY9x6g|
00000640 00 60 97 f2 1b 00 00 00 00 00 03 00 00 00 00 00 |.`..............|
00000650 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00000660 00 00 00 00 00 00 00 00 00 10 00 00 00 00 00 00 |................|
00000670 00 f0 02 00 00 00 00 00 00 00 95 f2 1b 00 00 00 |................|
00000680 00 60 02 00 00 00 00 00 00 00 00 00 00 00 00 00 |.`..............|
00000690 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
00001000 f7 a5 f9 a0 20 4c 56 4d 32 20 78 5b 35 41 25 72 |.... LVM2 x[5A%r|
00001010 30 4e 2a 3e 01 00 00 00 00 10 00 00 00 00 00 00 |0N*>............|
00001020 00 f0 02 00 00 00 00 00 00 06 00 00 00 00 00 00 |................|
00001030 a6 03 00 00 00 00 00 00 ea 36 42 05 00 00 00 00 |.........6B.....|
00001040 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
00001200 56 47 5f 54 45 53 54 20 7b 0a 69 64 20 3d 20 22 |VG_TEST {.id = "|
00001210
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So there's usable space between 0x0000 and 0x0600 (1536 bytes). There's also space between 0x0690 and 0x1000 (2416 bytes) although I doubt it's "safe" to use that for boot code.
So I just installed Grub 2 and took a look at what I needed:
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/lib/grub/i386-pc/lnxboot.img = 1024 bytes
/lib/grub/i386-pc/ext2.mod = 5524 bytes
/lib/grub/i386-pc/lvm.mod = 5496 bytes
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Some other interesting ones:
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/lib/grub/i386-pc/ata.mod = 7972 bytes
ata.mod.gz = 3963 bytes
/lib/grub/i386-pc/gzio.mod = 7740 bytes
ext2.mod.gz = 2968 bytes
lvm.mod.gz = 2896 bytes
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Definitely a no go...bummer this was a fun mini-project. About the only steps I could take from here would be to use a slightly smaller LVM and embed grub code at the end of the disk or create a loopback device with an offset to make space for grub code. BOTH of these solutions are a bigger "kludge" than Mr. Opinionated suggests GPT is 
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"The philosophy of one century is the common sense of the next." - Henry Ward Beecher |
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