[ale] raid suggestions

[Lightner, Jeff]

Another benefit to controller based is you can often manipulate things during POST when you get to the controller's BIOS.

On the flip side the downside is that you are often required to manipulate only that way so have to reboot to change things.   But then again I seldom change RAID setups after initial install.





-----Original Message-----
From: ale-bounces at ale.org [mailto:ale-bounces at ale.org] On Behalf Of Pat Regan
Sent: Thursday, February 09, 2012 3:53 PM
To: ale at ale.org
Subject: Re: [ale] raid suggestions

On Thu, 9 Feb 2012 20:37:03 +0000
"Lightner, Jeff" <JLightner at water.com> wrote:

> Hardware based (for true RAID) because it doesn't require CPU cycles.

I'm not sure what your definition of "true RAID" is.  As for software
RAID requiring more CPU cycles, though, the number of cycles required
is quite negligible on anything approaching a modern processor.

Here's how fast the kernel says my laptop can crunch the numbers for
RAID 6:

[428688.926111] raid6: int64x1    793 MB/s
[428688.993945] raid6: int64x2   1052 MB/s
[428689.061905] raid6: int64x4    713 MB/s
[428689.129830] raid6: int64x8    695 MB/s
[428689.197810] raid6: sse2x1    2263 MB/s
[428689.265749] raid6: sse2x2    2649 MB/s
[428689.333668] raid6: sse2x4    2941 MB/s
[428689.333675] raid6: using algorithm sse2x4 (2941 MB/s)

That is just shy of 3 GB per second and I'm pretty sure that these
calculations are single threaded.

CPU use isn't the performance argument to make against software raid,
though.  Bus bandwidth is usually a bigger factor.  Mirroring requires
almost zero cpu but it requires double the bandwidth.  This was a big
deal when your PCI bus was limited to only about 130 MB/s.

Today that isn't usually worth worrying about.  Even a single PCIe lane
has enough bandwidth for a pretty large number of drives...

Pat
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