[Bug 878582] [NEW] grub-probe fails on flashcache /dev/mapper root device

[Graeme]

Public bug reported:

When running root off of flashcache
(https://github.com/facebook/flashcache), which creates a device-mapper
virtual dev such as /dev/mapper/fc-root, grub-probe (and thus update-
grub) fails to find the root fs:

root at galvatron:~# grub-probe /
grub-probe: error: cannot find a GRUB drive for /dev/mapper/fc-root.  Check your device.map.
root at galvatron:~# grub-probe /dev/mapper/fc-root 
grub-probe: error: cannot stat `none'.
root at galvatron:~# ls -l /dev/mapper/fc-root 
lrwxrwxrwx 1 root root 7 2011-10-19 16:31 /dev/mapper/fc-root -> ../dm-0
root at galvatron:~# grub-probe /dev/dm-0 
grub-probe: error: cannot stat `none'.

Unfortunately, with this configuration, it seems to just ignore the
/boot/grub/device.map file. I've got the following in there, and the
error persists:

(hd0)   /dev/mapper/fc-root

Is there a way to have update-grub run without using grub-probe, to
avoid this error? Or a way to get grub-probe to correctly recognize
device-mapper devices?

ProblemType: Bug
DistroRelease: Ubuntu 11.04
Package: grub-pc 1.99~rc1-13ubuntu3
ProcVersionSignature: Ubuntu 2.6.38-11.50-generic 2.6.38.8
Uname: Linux 2.6.38-11-generic x86_64
NonfreeKernelModules: nvidia
Architecture: amd64
Date: Wed Oct 19 16:47:26 2011
ProcEnviron:
 LANGUAGE=en_CA:en
 PATH=(custom, user)
 LANG=en_CA.UTF-8
 SHELL=/bin/bash
SourcePackage: grub2
UpgradeStatus: Upgraded to natty on 2011-05-25 (147 days ago)

** Affects: grub2 (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: amd64 apport-bug natty

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https://bugs.launchpad.net/bugs/878582

Title:
  grub-probe fails on flashcache /dev/mapper root device

Status in “grub2” package in Ubuntu:
  New

Bug description:
  When running root off of flashcache
  (https://github.com/facebook/flashcache), which creates a device-
  mapper virtual dev such as /dev/mapper/fc-root, grub-probe (and thus
  update-grub) fails to find the root fs:

  root at galvatron:~# grub-probe /
  grub-probe: error: cannot find a GRUB drive for /dev/mapper/fc-root.  Check your device.map.
  root at galvatron:~# grub-probe /dev/mapper/fc-root 
  grub-probe: error: cannot stat `none'.
  root at galvatron:~# ls -l /dev/mapper/fc-root 
  lrwxrwxrwx 1 root root 7 2011-10-19 16:31 /dev/mapper/fc-root -> ../dm-0
  root at galvatron:~# grub-probe /dev/dm-0 
  grub-probe: error: cannot stat `none'.

  Unfortunately, with this configuration, it seems to just ignore the
  /boot/grub/device.map file. I've got the following in there, and the
  error persists:

  (hd0)   /dev/mapper/fc-root

  Is there a way to have update-grub run without using grub-probe, to
  avoid this error? Or a way to get grub-probe to correctly recognize
  device-mapper devices?

  ProblemType: Bug
  DistroRelease: Ubuntu 11.04
  Package: grub-pc 1.99~rc1-13ubuntu3
  ProcVersionSignature: Ubuntu 2.6.38-11.50-generic 2.6.38.8
  Uname: Linux 2.6.38-11-generic x86_64
  NonfreeKernelModules: nvidia
  Architecture: amd64
  Date: Wed Oct 19 16:47:26 2011
  ProcEnviron:
   LANGUAGE=en_CA:en
   PATH=(custom, user)
   LANG=en_CA.UTF-8
   SHELL=/bin/bash
  SourcePackage: grub2
  UpgradeStatus: Upgraded to natty on 2011-05-25 (147 days ago)

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