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Re: Software RAID and Fakeraid


From: Phillip Susi
Subject: Re: Software RAID and Fakeraid
Date: Thu, 02 Dec 2010 22:15:55 -0500
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.12) Gecko/20101027 Thunderbird/3.1.6

On 12/02/2010 08:36 PM, Neil Brown wrote:
If the array uses 0.90 or 1.0 metadata and comprises whole-disks (not
partitions), and if the array is RAID1, then each device (except for the very
end) contains exactly the same data as the whole array.
If you install grub to the array, then it will be installed onto all of the
(active) devices in the array.  And that is certainly the easiest way to
write to all device.

It won't write to 'spares', so if you want to be able to boot from spares as
well .... but I'm not sure that makes sense anyway.

Yes, for a raid1 with no spares, installing to the array is equivalent to installing to each individual disk, but it helps avoid confusion to ignore this fact and remain thinking in terms of the physical disks, at least as they appear to the bios.

Completely agree.  As I said, there are only some cases where you can boot
from an array which uses whole-disks.
One case if in the bios understands the array, such as Intel bios's with IMSM
metadata, or possibly some bioses with DDF metadata.
Another case is RAID1 which starts at the beginning of the device, where the
bios doesn't need to know about the RAID.

So how do we tell the difference? Right now grub uses the rule of dmraid = bios aware, so install to the raid device, and mdadm = software raid, so install to the component devices individually. You have noted that in same cases both methods will produce the same results, but grub needs to be certain that whichever method it chooses will work, whether or not either one will. To do this, it needs to install to the raid device if and only if it is a bios recognized fakeraid.



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