Most SSDs have two speed ratings for reading as well as for writing. The first rating is the sustained MB/s performance, which is the main marketing most manufacturers use. The second rating is the 4K Random IOPS performance, which gives a much better idea of how the drive will perform in the real world. The sustained MB/s rating is the sequential transfer rate the SSD will maintain continuously, such as over a period of 30 seconds. The 4K Random IOPS on the other hand is how many 4K (4096 byte) operations the drive will handle per second with each block being read or written to a random position.
Usually when a Random 4K IOPS figure is given, it may state that this is at a certain queue depth, such as 4, 16, 32 or 64. With a queue depth of 4, this means that there are 4 separate threads taking place with the drive, each thread independently running its own transfers. With the use of Native Command Queuing (NCQ), the SSD can handle these threads simultaneously to improve the overall throughput compared to running a single thread. While many hard disks use native command queuing to line up read/write operations to minimise seek times between each read/write operation, SSDs can read from and write to multiple NAND cells simultaneously, where as the read/write head in a hard disk can only be in one physical place at any time.
While it is nice to see how many IOPS a drive is capable of, it is also useful to see how this translates into actual throughput or even vice versa. For example, most benchmark tools such as CrystalDiskMark and AS SSD report the random 4K performance in throughput, i.e. MB/s, while the SSD’s specifications usually rates the 4K performance in IOPS.To see how to translate MBps into IOPS and vice versa, we need to do a little math:
IOPS = (MBps Throughput / KB per IO) * 1024
MBps = (IOPS * KB per IO) / 1024
So let’s say we have an SSD claiming a Random 4K write speed of 20,000 IOPS and it achieves 76.2MB/s in the CrystalDiskMark with the QD32 write test.
To convert the 76.2MB/s to IOPS, we perform the following calculation:
IOPS = (76.2 / 4) * 1024
IOPS = 19.1 * 1024
IOPS = 19,558.4
To see what throughput we need to achieve to match the actual 20,000 IOPS claim, we can perform this calculation in reverse:
MBps = (20,000 * 4) / 1024
MBps = 80,000 / 1024
MBps = 78.125MB/s
Note that each manufacturer uses its own method of coming up with their SSD IOPS ratings. Besides separate IOPS ratings for read and write speeds, a given SSD can behave quite different depending on the type of data being read or written, as well as the duration this transfer takes place. For example, an SSD that achieves 5,000 Random 4K IOPS sustained write over a period of 30 seconds may only achieve 1,000 IOPS sustained write over a period of 5 minutes. For SSDs using the SandForce processor, the compressibility of the data also has an impact, so two benchmark tools may show completely different IOPS readings if one tool sends uncompressible data and the other tool sends highly compressible data and also if one runs the benchmark for longer duration than the other.