Performance Comparisons – Sequential and Random

For sequential writes, we first note that for burst transfers, most SSDs have a flat response here, meaning throughputs are constant regardless of queue depth. With that out of the way, the four SBX capacities make for a decent throughput spread (more on that on the next page), with the SATA 860 EVO beating the 128GB SBX in write throughput.

The bulk of these products (all SBX and BPX, plus the 860 EVO) start at relatively low throughputs at QD=1. Fortunately, sequential transfers don't typically stick with QD=1, so we can look higher up the scale. That said, the 960 EVO does crush everything else here.

NAND SSDs are surprisingly fast at low QD random writes. This is for a few reasons. To explain better, let's review what happens when a typical NAND-flash SSD writes or reads:

  • Writes: Host sends data to SSD. SSD receives data and acknowledges the IO. SSD then passes that data onto the flash for writing. All necessary metadata / FTL table updates take place.
  • Reads: Host requests data from SSD. SSD controller looks up data location in FTL, addresses and reads data from the appropriate flash dies, and finally replies to the host with the data, completing the IO.

Aside from a necessary ramp up to maximum IOPS, typically by QD=8, we see that the random writes roughly match the sequential write throughputs seen earlier.

…and now we flip the script. Random reads require all of the work to be done before the IO can be completed, and the limit here at the more critical lower queue depths is NAND's read response time. NAND is the storage medium of all SSDs in this comparison, so we find them all starting at a similar range, with the ramp determined more by how many parallel flash dies are present.

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