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12Gb/s and 6Gb/s Smart Array Controllers - Conflicting specifications? Limits?

 
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Frequent Advisor

Re: 12Gb/s and 6Gb/s Smart Array Controllers - Conflicting specifications? Limits?

Hi @DanRobinson !!

Not sure it's ever made clear in this (rather long) thread.

The Drive Cage Backplane has to support the speed you want, along with the controller.

The DL580 Gen8 came out much later than the others (DL380, ML350, etc) and so it's drive cages all support 12G.

But dropping a P430 into a ML350 means the drive cage is now the bottleneck and causes the drives to be only 6G capable.

[Underline and bold is my emphasis]

Wow, I never would have thought that the drive cage is the limiting factor in transfer speeds! So I can get a P430 or a P420 external card and I could use 12G or 6G drives ... my maximum theoretical transfer speed is only going to be 6Gb/s on my ML350p Gen8 machine.

On the one hand, I'm a little disappointed because I really wanted that extra speed. However, on the other hand, I'm relieved because it's one less thing for me to worry about. I'm just going to have 6Gb/s speed so I can move on to other things.

I did notice when playing with RAID 0 arrays I did not get faster performance after going beyond two drives. Going to three drives and beyond made no difference. I probably reached the transfer speed limit of the cage.

FYI, the P430 is effectively a P440 Gen9 controller that was backported to Gen8.
So even though the Link Speed may be limited to 6G, the RAID 5/6 speed of the P430 should me much faster than the P420 due to it's use of the faster ASIC under the hood.

Lastly, if you use Spinning Rust, the 6G speed will always be faster than the drive can sustain anyway, beyond the initial cache hits from the RAM on the drive.  Fastest spinning drives only ever hit 250MB/s.
So it's only SSDs where this becomes an issue at all.

"Spinning Rust" almost has a romantic ring to it! Yes, I am using good old fashioned spinning drives in my project. This is another case of being saddened by the limit but relieved that I shouldn't worry about what I can't change. Just keep building.

Thanks again for this info!


Boris