Open Stack

Hi John,

On 1/19/2016 5:28 PM, John van Ommen wrote:
> I have a client who isn't happy with the performance of their storage.
> The client is currently running a mix of SAS HDDs and SATA SSDs.
>
> They wanted to remove the SAS HDDs and replace them with SSDs, so the
> entire array would be SSDs.
>
> I was running benchmarks on the current hardware and I found that the
> performance of the HDD array was close to the performance of the SSD
> array.
>
> To me, this indicates that we're reaching the limits of the controller
> that it's attached to. (An LSI RAID controller that's built into the
> system board.)
>
> I was about to recommend that they add a controller, when I realized
> that we may be reaching the limits of the PCI-E bus itself.
>
> Before I go and make a bad recommendation, I have a few questions:
>
> 1) Am I correct in assuming that the RAID controller, though
> physically on the system board, is still running through the PCI-E
> bus, just as if it was plugged into a slot?
Yes. In pre Intel SandyBridge architecture like Westmere processors,
there used to be a bridge chipset that extends the PCIe lanes to addone
cards, while in Sandybridge and onwards PCIe lanes comes straight from
the processor.

> 2) Am I correct in assuming that the limit for the PCI-E bus (version
> 2) is 500Mb/s? (https://en.wikipedia.org/wiki/PCI_Express)
I think you meant to say 500MB/sec per lane. Now what does the
motherboard say about the RAID controller, is it x4 or x8 ? So you can
multiply lane speed to x4 or x8. If x8 then using a PCIe Gen 2.0 bus, it
should provide 8 x 500 = 4000MB or 32Gbps. Whether the RAID card can
provide this much traffic or not that's another thing.

> And if points one and two are correct, is my hypothesis that adding
> more SSDs won't improve things true?
>
> Right now my benchmarks are showing that sequential reads are hitting
> about 600Mb/s. (I haven't confirmed if their server is PCI-E 2.0 or 3.0)
A single Samsung SSD 850PRO claims reads to be close to 550MB/sec. There
could be multiple factors affecting performance involving RAID card,
PCIe bus, RAID settings, cpu affinity etc.

-Azher

>
> John
>
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