Re: bootable RAID, number of member disks limitation

[Jordan Uggla]

On Thu, Jan 3, 2013 at 11:42 PM, Chris Murphy <address@hidden> wrote:
>
> On Jan 3, 2013, at 3:00 PM, Vladimir 'φ-coder/phcoder' Serbinenko 
> <address@hidden> wrote:
>
>> GRUB has no limitation on number on devices other than free memory to
>> hold structures and 2^32 for IDs. If there is any other limit it's a bug
>> and please file a bug report with the images in question (no need to
>> install, just use grub-fstest) However BIOS limits the number of disks
>> that can be accessed. Theoretically there could be up to 128 (or 256 if
>> you use floppy numbers as well) accessible through BIOS. The convention
>> is to use only 16 possible IDs for HDD (0x80-0x8f). In practice many
>> BIOSes are limited to less (8 is usual limit).
>
> I'm totally unclear on how to use grub-fstest.

I don't have a GNU/Linux system (other than my phone) to test this
with at the moment, but I think "grub-fstest --diskcount=11
/dev/sd{a..k} ls -- -l" should list all 11 disks, and their
partitions, with information about the filesystem each contains. If
(loop0,msdos1) is one of the devices that makes up this btrfs volume
then you should be able to list files on the filesystem with:

grub-fstest --diskcount=11 /dev/sd{a..k} ls '(loop0,msdos1)/'

>
> Should ls at grub rescue report all attached devices?

Yes, if it doesn't then that's a pretty clear indication that your
boot firmware can't properly handle many drives. You can also try
"grub-install --disk-module=native /dev/sdX" to test if grub can
access all of the drives using its own native disk drivers rather than
relying on the boot firmware.

>
> With single disk btrfs only (no md RAID), at a grub command prompt, ls reports
> (hd0) (hd0,msdos1) (hd1) (hd2) (hd3)
>
> Yet there are 11 devices attached. This is VirtualBox, so this very well may 
> be a vbox limitation. I haven't tried it with KVM's SEABIOS yet.


I am surprised to hear that this problem is appearing with VirtualBox
rather than real hardware. If this is a bug in VirtualBox's BIOS then
a bug report should definitely be filed with them.

-- 
Jordan Uggla (Jordan_U on irc.freenode.net)