[Bug 497039] Re: Fails to install on fully degraded RAID10

[Andrew Cranwell]

Is this actually a bug with grub, a bug with mdadm or an intended
design?

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Title:
  Fails to install on fully degraded RAID10

Status in “grub2” package in Ubuntu:
  New

Bug description:
  Binary package hint: grub2

  Hi,

  I was reinstalling Ubuntu on LVM over mdadm RAID10, this failed
  because the array was too degraded (missing 2 disks out of 4).

  My full device config was on the previous install:
  /boot on /dev/md0 == RAID1(sda1 + sdb1)
  / on LVM LV "root" in VG vg0 backed by single PV /dev/md1 == RAID1(sda1 + sdb1)

  sda and sdb were partitioned using regular PC partitions

  
  New install was towards:
  / on LVM LV "root" in VG fox-raid10 backed by single PV /dev/md2 == RAID10(sdc2 + missing + sdb2 + missing)

  sdc and sdd were partitioned using GPT with a bios_grub partition at
  the start for core.img

  
  I'm not sure that's the first error I've hit, but trying to debug why grub-install was failing, I got errors from grub-probe which I could reproduce with:
  grub-probe --device-map=/boot/grub/device.map --target=fs --device /dev/mapper/fox--raid10-root
  it failed with "error: unknown filesystem"

  I rebuilt grub2 from bzr tip (/grub/trunk/grub r1938) and reproduced
  the error. Poking in gdb I would see that md2 would never be iterated.
  Poking at grub_raid_iterate(), I found that the array was skipped
  because is_array_readable() returns false.

  The logic in is_array_readable() will return false at this test:
    if (array->nr_devs >= array->total_devs - n)
      return 1;
  in my case, array->nr_devs is 2, array->total_devs is 4, and with raid10 and my layout of n2 (the current default for raid10), n equals 1.

  So it seems GRUB2 considers the array broken, but it's in fact a
  working degraded config, where I lost all mirrors on the same stripe.

  Thanks,

  ProblemType: Bug
  Architecture: amd64
  Date: Tue Dec 15 17:29:40 2009
  DistroRelease: Ubuntu 9.10
  LiveMediaBuild: Ubuntu-Server 9.10 "Karmic Koala" - Release amd64 (20091027.2)
  NonfreeKernelModules: nls_iso8859_1 hfsplus hfs raid10 raid1 xfs exportfs reiserfs jfs ntfs vfat fat intel_agp atl1 mii nls_cp437 isofs fbcon tileblit font bitblit softcursor vga16fb vgastate vesafb usb_storage usbhid floppy ohci1394 ieee1394
  Package: grub-pc 1.97~beta4-1ubuntu4.1
  ProcEnviron:
   PATH=(custom, no user)
   SHELL=/bin/sh
  ProcVersionSignature: Ubuntu 2.6.31-14.48-generic
  SourcePackage: grub2
  Uname: Linux 2.6.31-14-generic x86_64

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