SSD vs. HDD Pricing: Seven Myths That Need Correcting - Page 4


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The differences and trends in performance are even more significant than cost. Consumer SSDs all seem to have maximum sequential read performances of about 500 to 550 MB/s and write performances up to 540 MB/s while random 4k IOPS never really exceed 100,000 for reads and range from 25,000 to 90,000 for writes.

On the other hand, enterprise SSDs using SAS have sequential read performances ranging from 430 MB/s to 925 MB/s and writes from 175 MB/s to 595 MB/s with random 4k IOPS (I/O request per second) as stated above. Enterprise SSDs that use a PCIe interface have by far the best performance with individual SSDs that usually have sequential read performance in excess of 3 GB/s and write performance between 630 MB/s and, in one special instance, 3.2 GB/s. Random 4k IOPS for PCIe enterprise SSDs could reach up to 750,000 for reads and 275,000 for writes, with most having IOPS of between 100,000 to 400,000 for reads and 65,460 to 200,000 for writes.

The consumer SSD products have very little differentiation among them and actually share similar performance numbers as well as price, while enterprise SSDs vary greatly. It should be noted that no real application can really take advantage of SSDs that support over 100,000 IOPS.

The status of consumer SSDs for both price and performance suggest a saturated market with little technological differentiation where total prices and price for capacity ($/GB) dictate the market, artificially lowering prices. The wide disparities in enterprise SSDs show a different market where products are specialized for specific workloads, environments or demands.

The general drop-off of write performance from consumer to enterprise SSDs against read performance strongly suggests that enterprise SSDs are incorporating additional error correction code (ECC) and other measures to ensure that data is written to the drive more reliably.

In fact, only one drive, the Super Talent RAIDDrive II Plus, actually offered greater write performance than read performance as it was designed for write heavy workloads and given a special caching system. This is also the only instance where write performance saw a significant increase from a predecessor system (the original RAIDDrive), otherwise, consumer SSD write performance has seen only slight increases, while enterprise SSDs have either remained constant or even decreased by as much as 50 percent to maintain data reliability.

These differences are important to consider when making forecasts regarding the use of SSDs in enterprise environments. The enterprise SSD environment is less predictable—and much more expensive—than the consumer SSD market generally used for SSD forecasts.

Reality: The Reduced Quality

Even if economic forces are favorable to continuing price reductions of SSDs and NAND flash, a 2012 study by Microsoft Research (PDF) has found that a dilemma arises when trying to increase density and reduce cost of SSDs. The study looked at 45 flash chips from six different manufacturers and found that, as density increases, bit error rate (BER) and performance decrease. This is because the number of ranges of electrical charges necessary to store data on a single cell increase as densities increase.

The researchers found that, as feature size decreases (increasing density), bit error rates increase. While the SLC and MLC chips with cells that had feature sizes of between 80 and 60 nanometers (nm) usually had BERs of 1e-08, those with feature sizes of 40 nm had BERs at or below 1e-07, and the TLC chips with feature sizes of 20 nm had BERs of, at best, 1e-03.

In addition, researchers also found that increasing density also increases read and write latencies. NAND chips with feature sizes above 64nm had read latencies of 20us or less and write latencies of 0.5ms or less, while those with feature sizes of 32nm or less had read latencies between 20us and 60us and write latencies between 0.5ms and 2.5ms.

This leaves SSD and NAND manufacturers with a choice among density, cost, reliability, and performance. In any scenario, at least one of these four must be sacrificed to improve the others. This means that, even if SSDs can achieve cost parity with HDDs, it will be at the expense of reliability, density or performance. In fact, as discussed above, enterprise-grade SSDs already sacrifice write performance, cost and even density to address the threat of reduced reliability and data integrity and have built non-2.5” form factor configurations and added special coding or technologies to meet reliability and performance demands, resulting in more costly products.

Reality: Power Consumption

The last key assumption in many arguments is that SSDs present a much more efficient alternative to disks in regards to power consumption and that they will become more important because database and supercomputing systems will hit a power consumption barrier of about 20 MW. Some point to the fact that SSDs at idle save far more power than HHDs constantly spinning, meaning that SSDs offer a cheaper option in the long run as they will use up less power.

The reality is that, currently and into the near future, this issue is not as clear as some would like to think.

The Seagate Enterprise 15K 2.5” form factor HDD and Terascale HDD have power consumption needs of 1 W and 6.5 W per drive, respectively. However, SSDs are far more varied. Consumer SSDs, designed for laptops or tablets, often have power consumptions of between 0.1 and 1.5 W per drive, however enterprise SSDs can range from 3 W to 30 W depending on make and model with most falling between 3 W and 10 W.

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