Results: 128 KB Sequential Performance Scaling In RAID 0
128 KB Sequential Read Performance Scaling in RAID 0
Testing with 128 KB sequential reads, we get almost 2 GB/s from the four-drive array and more than 4.2 GB/s using 24 of the SSD DC S3700s. If we were getting an even 500 MB/s per drive, as Intel specifies, the 24x array would yield around 12 GB/s. Each of our Intel controller cards uses eight third-gen PCI Express lanes, so each one should be able to push more than 4,000 MB/s.
Still, we're seeing a massive amount of throughput. It's almost like reading an entire single-layer DVD every second. And we can do that speed from the first LBA to the last because we're not relying on any caching. This is all-flash performance.
128 KB Sequential Write Performance Scaling in RAID 0
We get even better performance with writes. The 24x array encroaches on the 5 GB/s mark, falling just short at 4.8 GB/s. The 16x and 8x configurations group together around 3 GB/s, while the four-drive array backpedals just a few percent compared to its read numbers.
When Keepin' it Real Goes Wrong
Make no mistake; these are breathtakingly awesome numbers. But it's hard to shake the feeling that something is robbing us of achieving epic, face-melting benchmark results.
So, what gives, then?
Could it be the strip size we chose for these RAID 0 arrays? No. After extensive testing, we settled on 64 KB chunks. Each 128 KB transfer is serviced by two drives, since 128 KB divided by two equals 64 KB, our chunk size. With enough parallel requests, everything should be good to go.
As we saw on the last page, we still get great scaling with 4 KB random transfers. So, it's probable that we're encountering a throughput issue. Since each drive is capable of large sequential transfers in excess of 400 MB/s, the issue is most pronounced on this page. The SSD DC S3700s can't put out more than 300 MB/s in our 4 KB random testing, and we expect to lose much of that anyway. So it makes sense that we'd run into bandwidth-sapping limitations here, and not there.
After investigating the RMS25KB/JB IR modules, we discovered that they were running in full PCI Express 3.0 mode and fully capable of pushing data back and forth from our SSD DC S3700s with minimal performance impact. As it happens, the culprit is the one thing we really need: our 24-bay SAS/SATA backplane.
Sad but true. Twenty-four total bays are enabled by a trio of eight-drive bays grafted to our server's exterior. Each possesses two SAS 8087 ports (one for every four drives), and they're just not able to get data through unmolested. Whether any backplane would work in this situation is uncertain, and bypassing ours simply wasn't an option for today's experiment.
