Conclusion: Second-Generation 6 Gb/s Systems Needed
An armada of 16 SSDs combined in a fast (albeit risky) RAID 0 arrangement would probably not be something many users would set up in reality, neither in the corporate world nor in general use. Then again, it was not our goal to create a realistic test scenario, but to find out just how much power we could tease out of a bunch of decent flash SSD drives a year after our first attempt at doing the same thing.
So, here's what we found: the rebirth of our extreme RAID, this time comprised of MLC-based Samsung 470 SSDs instead of more expensive SLC-based Intel X25-Es, generates throughput of more than 3 GB/s and I/O performance at up to 170 000 operations per second. However, the high read/write speeds are not as constant as we expected. Just like the I/O performance levels, they unfold with increasing queue depth, reading their peak values beyond 3 GB/s only in sequential measurements.
We expected the LSI MegaRAID 9260-8i and MegaRAID 9280-24i4e RAID controllers to perform admirably, which they did. But we did not expect them to quietly bottleneck our performance ambitions so severely. The manufacturer claims each controller can manage a maximum of 150 000 IOPS and around 1600 MB/s, which seems like quite a lot. This is actually true for most conventional setups, so long as you don’t go for extreme performance like we are here. Mathematically, 16 SSDs at up to 261 MB/s each could jump over the 4 GB/s line, which is impossible with current hardware. We want to make clear that this isn’t an issue of 3 Gb/s signaling versus 6 Gb/s, but rather a matter of bandwidth and I/O capabilities on the host system side, and, more important, the RAID controller. It all goes to show how important balance is in your own high-end storage. It's not hard to conceive of a similar bottleneck affecting daisy-chained JBODs.
The RAID story is certainly far from over. For one thing, the next generation of SAS-capable RAID controllers will provide much higher bandwidth and I/O capabilities. New products, such as LSI’s recently-announced 9285-8e/i, which we’ll be looking at soon, come with 6 Gb/s interfaces. At the same time, the latest 6 Gb/s SSDs demonstrate impressive throughput, which shows that there still is a lot of room to grow performance. Expect a new extreme RAID SSD project to follow up later this year.
