Re: RAID-1 to RAID-5 online migration?
On Mon, 13 Sep 2004 05:20, Adrian 'Dagurashibanipal' von Bidder
<avbidder@fortytwo.ch> wrote:
> > Machines that can handle such an IO load have faster CPUs. So for any
> > but the very biggest machines there is no chance of CPU performance being
> > a problem for RAID-5.
>
> You certainly have more experience than I - I was thinking about machines
> where the CPU is already heavily loaded by userspace tasks, where the
> additional load from RAID5 might be a poblem. Don't know for certain,
> though.
If you have a machine that is capable of 1296MB/s for RAID-5 calculations (as
my P3-650 is), and if that machine has disk IO capacity of 200MB/s (not the
maximum that you could achieve with such hardware, but better than the vast
majority of such machines) then in theory you will use 15% of your CPU time.
But considering that a bus-mastering hardware RAID device will take some RAM
access time away from the CPU and that the RAM bandwidth is often the CPU
performance bottleneck it's quite likely that a large portion of that 15% CPU
performance hit will happen no matter how you attach your disks.
It would be really useful if someone spent a couple of weeks benchmarking
these things and wrote a magazine article about it.
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