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SSD Throughput, Latency and IOPS Explained – Learning To Run With Flash


as-ssd Hitachi 2TB Z97 Win 8.1 MBs 6.26.2014as-ssd SanDisk EX II Z97 Win8.1 MBs 6.23.2014







If we check the speed difference (MB/s) we see it is significant but not tremendous, 149.86 MB/s for the HDD, vs 514.28 MB/sL the SSD is 3.4 times faster. Yes, that’s the same difference as the IOPS; all these things are related. Large file sequential IO is the easiest for any storage device, which is why we see sequential speed specifications in marketing information. It is also easy to understand. The truly important indications of performance are not found here.

The 4K read speed of the HDD is surprisingly small, 0.69MB/s compared to the SSD’s 36.79MB/s, and again is over 53 times faster in the SSD. A 4K write speed for the HDD of 1.22 MB/s is better than the read speed, but again dwarfed by the SSD at 128.65 MB/s, and again over 105 times faster.

AHCI and NCQ were originally designed for HDDs, to enhance their performance. Given that, the HDD’s results are not impressive. We see an increase in the IOPS to 405, or 2.3 times greater, with read speed increase of less then two times, to 1.58 MB/s. The SSD is capable of 92,586 read IOPS, over 228 times faster! Once again, the SSD’s read speed of 361.66MB/s is over 228 times faster than the HDD.

Unfortunately, the HDD gains nothing from NCQ in the NCQ 4K write test, staying at 311 IOPS and 1.21 MB/s. The SSD’s performance is over 193 times the write IOPS of the HDD, and at 235MB/s is over 194 times faster than the HDD.


The final row we’ll look at is Access time, which is latency. A single access time figure is very simplified, as it varies depending on many things, in this case it must be an average. The HDD’s Read Access Time of 15.785 millisecond sounds quick, and it is, but the SSD’s Access Time of 0.031 milliseconds is 509 times faster! Write access time is faster than the read on the HDD, since all it needs to do is get the address of some empty space. The SSD’s write access time is still over 102 times faster than the HDD’s. Do you think that extremely reduced access times like this will only be noticeable when a PC boots an OS?

Samsung 830 128GB Read WD10EZEX 1TB Read

Take a look at these two HDTune Pro results; the SSD on the left and the hard drive on the right.  Do you notice how the hard drive performance decreases as the data transfer continues, in comparison to the SSD that remains level? Now look at the Access Time of the hard drive compared to that of the SSD.  This is a perfect example of how a SSD, with very low latency compared to the HDD, moves data continuously like oil through a pipeline, whereas the HDD must continuously return for chunks of data, slowing performance and increasing disk access times significantly.


Throughput, latency and IOPS form the performance triangle of all SSDs, regardless of whether we are speaking of a $70 consumer SSD or a $15K PCIe enterprise SSD.  As much as throughput may be the key to retail sales while latency and IOPS are that of enterprise, this knowledge enables our understanding of why our PC starts in 15 seconds with a SSD, vice that of a minute or more with HDD.

Simply, IOPS are how often or fast the storage device can perform IO requests, latency describes how long it takes for an IO request to begin, and throughput is the actual speed of the data transfer and most often measured in MB/s. In all cases, we found that a SSD performs better than a HDD, from about 3.5 times to over 500 times faster.

HDD vs SSD-10

The two most important factors, the ability to perform more IO requests, and to start them faster range from about 50 to 500 times faster. We also saw that IOPS contain the same information that the MB/s speed does.

At the consumer level, most will never use the ultimate performance capabilities that an SSD offers very often, if ever.  Having a SSD in your system means higher performance, complete silence, a cooler system, much higher endurance and data safety should you drop your system, as well as longer battery life for laptops.  The fact that we forget these characteristics after getting used to the SSD is probably the ultimate compliment we can experience.


As fast as SSDs have found mainstream consumer use, they are unfortunately grouped in the same picture as a hard drive, if only for the fact that they are seen as storage and little more.  Both the size and demography of our readership paints this picture clearly, however, there exists an amazing opportunity to take our readers a step further; an opportunity to show why we might have such passion for what many might consider a very dry area.  In this segment of 'Learning To Run With Flash' let's gain an understanding of the 'Big 3' of SSD performance, these…

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