Re: Autostart RAID 1+0 (root)

[Jakob Østergaard]

On Tue, Jan 22, 2002 at 12:15:28PM -0500, Bill Davidsen wrote:
> On Fri, 18 Jan 2002, Alok K. Dhir wrote:
> 
> > I want to test using a software RAID 1+0 partition as root: md0 and md1
> > set up as mirrors between two disks each, and md2 set up as a stripe
> > between md0 and md1.  However, the RedHat 7.2 installer doesn't allow
> > creating nested RAID partitions.
> 
> Here's my understanding. If you are using hardware RAID you can do
> anything your controller supports, and it looks like a single drive to the
> CPU. But if you are looking for reliable boot, you need to use /boot as a
> RAID-1 partition on the first two drives, and make that partition the
> active partition (that may not be needed with your BIOS).

I think he is referring to software RAID. And yes, it is indeed a problem
that the RedHat installer cannot create nested RAIDs (at least, I too was
unable to do that, so either it's impossible, or I'm equally blind).

> This is because if the first disk fails totally, the 2nd will be used to
> boot. You also should use an initrd image to be sure all you need to get
> up is on that small mirrored partition. After that your other partitions
> can be whatever pleases you.

Also, GRUB/LILO only support booting from RAID-1 (or no RAID).

...
> > 
> > Does the kernel support autostarting nested RAID partitions?
> > 

Yes it does.  If you have persistent superblocks on all arrays, they
*should* autostart.

If you boot from the 4G disk, does the array start properly ?  Does
it start properly even if you remove your /etc/raidtab ?

Please check that you have the correct RAID levels either compiled
into your kernel, or on an initrd.

> > Is doing software 1+0 a bad idea anyway due to performance issues?
> 
> It should outperform most other RAID configs under heavy load, but in most
> cases RAID-5 is fine for system which don't need the absolute highest
> performance. Note that the extra writes are queued and there are no extra
> reads unless it is in recovery mode. RAID-1 can be faster, because there
> are two copies of the data, if one drive is busy the other can be used. I
> haven't checked to see that software RAID does that correctly and gets the
> benefit.

A performance improvement went into 2.4 at some stage - all newer 2.4 kernels
will schedule reads to the mirror which has it's head nearest to where the
read should occur.  This works very well in my experience.

-- 
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