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

[Tessa]

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

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[[e-mail] (mike-papersolve)]

Confirmed according to the flashcache docs, temporary solution is to edit grub.cfg manually (see https://github.com/facebook/flashcache/blob/master/README-DKMS):

Known Issues:
-------------
  - Right now grub-probe fails to detect the underlying root device when booting off flashcache,
    so you'll have to edit /boot/grub/grub.cfg manually.
 - Because of previous point, update-grub will likely fail.

[Marcus Tomlinson (marcustomlinson)]

This release of Ubuntu is no longer receiving maintenance updates. If this is still an issue on a maintained version of Ubuntu please let us know.

[Launchpad Janitor (janitor)]

[Expired for grub2 (Ubuntu) because there has been no activity for 60 days.]