[Gllug] Production system - Linux 2.4.24, LVM and cciss

[Peter Childs]

On Sun, 11 Jan 2004, Martin A. Brooks wrote:

> At 16:32 11/01/2004 +0200, you wrote:
> >Have been through the various RAID scenarios and RAID 1+0 has been the
> >plan from the start; RAID 5 is too slow.
>
> Almost by definition RAID5 is faster for both reading and writing than
> RAID0+1.  In cases where RAID5 is actually slower, it often means that the
> system was poorly thought out.  For example, small numbers of larger disks
> where larger numbers of smaller disks would been appropriate.
>
>
>
	Hmm Lets Finish this with some maths.... To keep things simple 4
Disks 80Gb Disks).

Raid 1+0 Write 2 Read 1
Raid 5	 Write 2 Read 1

	So its the same for the smallest number of disks you can have the
only difference is that the maths is heaver in Raid 5.

So Lets up that to 6 Disks

Raid 1+0 Write 3 Read 1
Raid 5   Write 2 Read 1

	So physically Raid 5 should be faster..... This gets better

8 Disks

Raid 1+0 Write 4 Read 1
Raid 5   Write 2 Read 1

	However if you are not writing all the time Raid 1+0 wins due to
the fact you don't have to read the right disk.

	On Failure Rate you lose either way if you don't want to lose more
than 1 disk as that 2nd disk could always be part of the same pair!

	Of course you can always through your numbers by using Raid 5+0.

Peter Childs

-- 
Gllug mailing list  -  Gllug at gllug.org.uk
http://lists.gllug.org.uk/mailman/listinfo/gllug