Performance Comparisons – Sequential and Random
The 970 PRO is faster than its predecessor, but for burst writes can't match the SLC cached write speeds of the 970 EVO 500GB and 1TB models. Do note that at QD2 and up, the WD Black still cleans house for sequential writes.
The 970 PRO offers impressive gains over the 960 PRO, especially at lower QD.
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.
The 960 SSDs are the green-overlapped-orange lines running near the bottom of the pack. The WD Black was impressive at the time, but the newer 970 products make an impressive jump in performance here.
…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.
For this chart, I've zoomed in a bit and shifted to a log scale so we can more clearly see the spread. It's still a close race, but the two PRO series drives clearly dominate over the rest of the pack. These differences are more clearly seen as we get into our QD weighted comparisons (next page, please).