Slashdot Mirror
Triple M.2 NVMe RAID-0 Testing Proves Latency Reductions
Vigile writes: The gang over at PC Perspective just posted a story that looks at a set of three M.2 form factor Samsung 950 Pro NVMe PCIe SSDs in a RAID-0 array, courtesy of a new motherboard from Gigabyte that included three M.2 slots. The pure bandwidth available in this configuration is amazing, breaching 3.3 GB/s on reads and 3.0 GB/s on writes. But what is more interesting is a new testing methodology that allows for individual storage IO latency capturing, giving us a look at performance of SSDs in all configurations. What PC Perspective proved here is that users often claiming that RAIDs "feel faster" despite a lack of bandwidth result to prove it, are likely correct. Measurements now show that the latency of IO operations improves dramatically as you add drives to an array, giving a feeling of "snappiness" to a system beyond even what a single SSD can offer. PC Perspective's new testing demonstrates the triple RAID-0 array having just 1/6th of the latency of a single drive.
I've been building PCs long enough to remember a time when things were improving so quickly that it made no sense to keep a computer for more than 4 years. But since then, the progress in CPU performance has reached a plateau. People like me, who bought a good Sandy Bridge system in 2011, still have a system that doesn't come close to feeling crippled and lazy. We don't have much reason to envy the people who bought the latest generation of i5/i7 systems. Five years used to mean an order of magnitude improvement in performance. Now it's not even a doubling. I've sometimes wondered when I will finally start feeling the urge to upgrade my system.
These SSD latency numbers are the first thing I've seen that gave me the feeling that there is some truly worthwhile trick that my present computer can't come close to matching. I'm not saying that I now want to upgrade, but on reading this, I have become upgrade-curious for the first time in many years.
These results seem to be very questionable. Their graphs claim that in some configurations almost all 4k read requests are handled within 100 ns. But getting even a single DRAM burst from a random DRAM location already takes almost 100ns, even through the memory controller is connected with a much tighter interface, optimized for low latency and PCIe is much slower than DRAM interface. Even without overhead 4 PCIe 3.0 lanes ( 8 GB/s) can only transfer 8 KB per s. Transfering a 4 KB Block should thus take at least 0.5 s or 500ns and that does not include any overhead nor the time needed to actually send the request to the SSD, open the page from the NAND flash, run ECC and decompression.
Jan
The SSD controller already does a form of this, as it is talking to multiple flash memory dies over multiple channels. RAID is just another layer to get even more performance out of more parallelism (and as we figured out in testing, to considerably drop the latency under load).
Allyn Malventano
Storage Editor, PC Perspective
this sig was brought to you by the letter