[Gllug] RAID on RAID

[Christian Smith]

On Wed, 3 Nov 2004, Rich Walker wrote:

>
>Since the list is quiet...
>
>I was pondering disk allocation strategies the other afternoon, and I
>suddenly realised I've allocated my root partition all wrong on the
>server...
>
>At the moment, the root partition is the only un-raided partition. There
>are 4 disks, on separate cables, with a bunch of partitions doing a
>bunch of stuff.
>
>The idea that occurred to me was to allocate a ~5GB chunk of each disk
>and then do
>hda1 + hde1 => md0, RAID1
>hdc1 + hdg1 => md1, RAID1
>md0 + md1 => md2, RAID1
>
>and then mount md2 as /


Urgh! If you want to make use of all four disks you'd be better off
having a RAID5 configuration. 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.


>
>Now, clearly write will be slow :-> But write to / is rare - most writes
>go to /home, /var, /tmp and some to /big.
>
>Reads should alternate between md0 and md1.
>
>If any one disk controller goes down, no problem.


Controllers would be the last thing to fail. They are solid state
electronic devices. You'd probably have problems with drives first, then
cables.


>If any three disks go down, no problem.


If you are planning for redundency to survive three disk failures at any
one point, you should have a new supplier you trust.


>
>But will LILO or GRUB cope with booting from RAID-on-RAID? And have I
>missed anything?
>
>cheers, Rich.
>
>

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