[Gllug] RAID on RAID

[Rich Walker]

Mike Brodbelt <mike at coruscant.demon.co.uk> writes:

> On Wed, 2004-11-03 at 16:44 +0000, Christian Smith wrote:
>
>> Urgh! If you want to make use of all four disks you'd be better off
>> having a RAID5 configuration.
>
> That depends on whether the intention is maximum redundancy or not. The
> scheme he's outlined can survive total failure of 3 disks, at the cost
> of sacrificing 75% of the usable space. 

Well, I was only thinking of using ~5-6GB of each for the /
partition. The rest actually goes either into another mirror for /home,
or into the big RAID5 storage pool.

> A RAID5 configuration can
> survive the loss of only a single disk, though he would only need to
> sacrifice 25% of the usable space. If you had more disks, you could go
> for a setup where you build multiple 2 disk mirror sets, 

This seems to be what is done for big database servers - one RAID-1 pair
per "access point", to maximise the number of spindles available.

> and then a
> large RAID 5 out of all your mirrors. That can give reasonable
> performance and high redundancy, but you have to think about multiple
> controllers and things as well.

>
>> So long as your initrd and lilo/grub boot
>> off a non-RAIDed boot partition you should be dandy to mount a RAID5 root.
>> Then you simply have multiple copies of /boot on different disks for
>> recovery if /boot is lost.
>
> You can make /boot RAID 1. Lilo can boot a kernel from RAID 1, and
> modern versions will even write the boot block onto both member disks,
> so the machine should remain bootable whichever disk fails.

But does it work through RAID1-on-RAID1? A quick glance through the
documentation suggests all sorts of interesting things - but not that
:-<

Any grub experts?

cheers, Rich.

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