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 > > -- > This message has been scanned for viruses and > dangerous content by *MailScanner* <http://www.mailscanner.info/>, and is > believed to be clean. > > > _______________________________________________ > Mailing list: http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack > Post to : [email protected] > Unsubscribe : http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack
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