Re: bootable RAID, number of member disks limitation

[Vladimir 'φ-coder/phcoder' Serbinenko]

On 03.01.2013 21:28, Chris Murphy wrote:

> I'm finding GRUB2 will boot from md RAID 0, 1, 4, 5, 6, with /boot in the 
> array, up to six member devices. At device 7, GRUB2 fails to boot the array, 
> drops to a grub rescue prompt saying the mduuid couldn't be found.
> 
> With Btrfs single and RAID 0 profiles, /boot on Btrfs is bootable for up to 4 
> devices. At the 5th device in a volume, it's no longer bootable by GRUB2, I 
> get a grub rescue prompt. I get variable results with RAID 10 for some reason 
> where often it works with 5 and 6 disks.


> 
> The question is if this is expected, as-designed, and if it's stable? I feel 
> like some subjective (certainly objective limits are OK) limit for number of 
> member devices is acceptable, but needs to be known, so that installers can 
> inhibit or warn users from creating unbootable arrays. And then how to 
> communicate this downstream?

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).

> 
> 
> Chris Murphy
> _______________________________________________
> Grub-devel mailing list
> address@hidden
> https://lists.gnu.org/mailman/listinfo/grub-devel
> 



-- 
Regards
Vladimir 'φ-coder/phcoder' Serbinenko

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