Won't boot if grub on partition holding the LVM root fs

[Stephane Chazelas]

$ apt-cache policy libblkid1
libblkid1:
  Installed: 2.19.1-2ubuntu3
  Candidate: 2.19.1-2ubuntu3
  Version table:
 *** 2.19.1-2ubuntu3 0
        500 http://gb.archive.ubuntu.com/ubuntu/ oneiric/main amd64 Packages
        100 /var/lib/dpkg/status

blkid will fail to report a partition as a "LVM2 Member" if it contains a grub boot sector.

On Ubuntu 11.10 where VGs are activated by udev upon finding LVM physical volumes (as opposed to "lvm vgchange -ay" being run unconditionally in earlier versions), that means that it won't boot as it won't find the root FS:

<pre>
Gave up waiting for root device. Common problems:
 - Boot args (cat /proc/cmdline)
   - Check rootdelay= (did the system wait long enough?)
   - Check root= (did the system wait for the right device?)
 - Missing modules (cat /proc/modules; ls /dev)
ALERT! /dev/mapper/VG_linux-ubuntu64 does not exist. Dropping to a shell!

BusyBox v1.18.4 (Ubuntu 1:1.18.4-2ubuntu2) built-in shell (ash)
Enter 'help' for a list of built-in commands.
(initramfs) blkid -o udev -p /dev/vda4
ID_PART_TABLE_TYPE=dos
ID_PART_ENTRY_SCHEME=dos
ID_PART_ENTRY_TYPE=0x8e
ID_PART_ENTRY_FLAGS=0x80
ID_PART_ENTRY_NUMBER=4
(initramfs) lvm vgchange -ay
  5 logical volume(s) in volume group "VG_linux" now active
(initramfs) lvm pvs
  PV VG Fmt Attr PSize PFree
  /dev/vda4 VG_linux lvm2 a- 771.30g 503.30g
</pre>

It can be reproduced with the PV on a partition or on a full disk such as with this script:

<pre>
#! /bin/sh -
set -ex
truncate -s 1T test
qemu-nbd -c /dev/nbd0 test
sleep 1
pvcreate /dev/nbd0
vgcreate V /dev/nbd0
: Before grub:
blkid -o udev -p /dev/nbd0
xxd -r << \EOF 1<> /dev/nbd0
0000000: eb63 9000 0000 0000 0000 0000 0000 0000 .c..............
0000010: 0000 0000 0000 0000 0000 0000 0000 0000 ................
0000020: 0000 0000 0000 0000 0000 0000 0000 0000 ................
0000030: 0000 0000 0000 0000 0000 0000 0000 0000 ................
0000040: 0000 0000 0000 0000 0000 0000 0000 0000 ................
0000050: 0000 0000 0000 0000 0000 0080 7eb0 0300 ............~...
0000060: 0000 0000 fffa 9090 f6c2 8074 05f6 c270 ...........t...p
0000070: 7402 b280 ea79 7c00 0031 c08e d88e d0bc t....y|..1......
0000080: 0020 fba0 647c 3cff 7402 88c2 52be 807d . ..d|<.t...R..}
0000090: e817 01be 057c b441 bbaa 55cd 135a 5272 .....|.A..U..ZRr
00000a0: 3d81 fb55 aa75 3783 e101 7432 31c0 8944 =..U.u7...t21..D
00000b0: 0440 8844 ff89 4402 c704 1000 668b 1e5c .@.D..D.....f..\
00000c0: 7c66 895c 0866 8b1e 607c 6689 5c0c c744 |f.\.f..`|f.\..D
00000d0: 0600 70b4 42cd 1372 05bb 0070 eb76 b408 ..p.B..r...p.v..
00000e0: cd13 730d f6c2 800f 84d8 00be 8b7d e982 ..s..........}..
00000f0: 0066 0fb6 c688 64ff 4066 8944 040f b6d1 .f....d.@f.D....
0000100: c1e2 0288 e888 f440 8944 080f b6c2 c0e8 .......@.D......
0000110: 0266 8904 66a1 607c 6609 c075 4e66 a15c .f..f.`|f..uNf.\
0000120: 7c66 31d2 66f7 3488 d131 d266 f774 043b |f1.f.4..1.f.t.;
0000130: 4408 7d37 fec1 88c5 30c0 c1e8 0208 c188 D.}7....0.......
0000140: d05a 88c6 bb00 708e c331 dbb8 0102 cd13 .Z....p..1......
0000150: 721e 8cc3 601e b900 018e db31 f6bf 0080 r...`......1....
0000160: 8ec6 fcf3 a51f 61ff 265a 7cbe 867d eb03 ......a.&Z|..}..
0000170: be95 7de8 3400 be9a 7de8 2e00 cd18 ebfe ..}.4...}.......
0000180: 4752 5542 2000 4765 6f6d 0048 6172 6420 GRUB .Geom.Hard
0000190: 4469 736b 0052 6561 6400 2045 7272 6f72 Disk.Read. Error
00001a0: 0d0a 00bb 0100 b40e cd10 ac3c 0075 f4c3 ...........<.u..
00001b0: 0000 0000 0000 0000 0000 0000 0000 0000 ................
00001c0: 0000 0000 0000 0000 0000 0000 0000 0000 ................
00001d0: 0000 0000 0000 0000 0000 0000 0000 0000 ................
00001e0: 0000 0000 0000 0000 0000 0000 0000 0000 ................
00001f0: 0000 0000 0000 0000 0000 0000 0000 55aa ..............U.
EOF
: After grub:
blkid -o udev -p /dev/nbd0
: clean-up
vgchange -an V
qemu-nbd -d /dev/nbd0
rm -f test
</pre>

Which gives:

<pre>
$ sudo sh ./reproduce
+ truncate -s 1T test
+ qemu-nbd -c /dev/nbd0 test
+ sleep 1
+ pvcreate /dev/nbd0
  Writing physical volume data to disk "/dev/nbd0"
  Physical volume "/dev/nbd0" successfully created
+ vgcreate V /dev/nbd0
  Volume group "V" successfully created
+ : Before grub:
+ blkid -o udev -p /dev/nbd0
ID_FS_UUID=1iWyMS-ZzMm-l7nv-kLTc-V2Rq-CqRo-q4RTnz
ID_FS_UUID_ENC=1iWyMS-ZzMm-l7nv-kLTc-V2Rq-CqRo-q4RTnz
ID_FS_VERSION=LVM2\x20001
ID_FS_TYPE=LVM2_member
ID_FS_USAGE=raid
+ xxd -r
+ : After grub:
+ blkid -o udev -p /dev/nbd0
ID_PART_TABLE_TYPE=dos
+ : clean-up
+ vgchange -an V
  0 logical volume(s) in volume group "V" now active
+ qemu-nbd -d /dev/nbd0
/dev/nbd0 disconnected
+ rm -f test
</pre>

[Stephane Chazelas (stephane-chazelas)]

Actually,

00001f0: 0000 0000 0000 0000 0000 0000 0000 55aa ..............U.

It's the MBR signature (the 55aa above) that causes problem. If I replace it with "0000", blkid sees it as a LVM member again but the system doesn't boot as its not recognised as a boot sector.

[Stephane Chazelas (stephane-chazelas)]

Also affects 12.04