Performance characteristics
The most common performance characteristics measured are sequential and random operations. Sequential operations access locations on the storage device in a contiguous manner and are generally associated with large data transfer sizes, e.g. ≥ 128 kB. Random operations access locations on the storage device in a non-contiguous manner and are generally associated with small data transfer sizes, e.g. 4kB.
The most common performance characteristics are as follows:
| Measurement | Description |
|---|---|
| Total IOPS | Total number of I/O operations per second (when performing a mix of read and write tests) |
| Random Read IOPS | Average number of random read I/O operations per second |
| Random Write IOPS | Average number of random write I/O operations per second |
| Sequential Read IOPS | Average number of sequential read I/O operations per second |
| Sequential Write IOPS | Average number of sequential write I/O operations per second |
For HDDs and similar electromechanical storage devices, the random IOPS numbers are primarily dependent upon the storage device's random seek time, whereas, for SSDs and similar solid state storage devices, the random IOPS numbers are primarily dependent upon the storage device's internal controller and memory interface speeds. On both types of storage devices, the sequential IOPS numbers (especially when using a large block size) typically indicate the maximum sustained bandwidth that the storage device can handle.[1] Often sequential IOPS are reported as a simple Megabytes per second number as follows:
(then converted to MB/s)
Some HDDs/SSDs will improve in performance as the number of outstanding IOs (i.e. queue depth) increases. This is usually the result of more advanced controller logic on the drive performing command queuing and reordering commonly called either Tagged Command Queuing (TCQ) or Native Command Queuing (NCQ). Many consumer SATA HDDs either cannot do this, or their implementation is so poor that no performance benefit can be seen.[citation needed] Enterprise class SATA HDDs, such as the Western Digital Raptor and Seagate Barracuda NL will improve by nearly 100% with deep queues.[4] High-end SCSI drives more commonly found in servers, generally show much greater improvement, with the Seagate Savvio exceeding 400 IOPS—more than doubling its performance.[citation needed]
While traditional HDDs have about the same IOPS for read and write operations, many NAND flash-based SSDs and USB sticks are much slower writing than reading due to the inability to rewrite directly into a previously written location forcing a procedure called garbage collection.[5][6][7] This has caused hardware test sites to start to provide independently measured results when testing IOPS performance.
Flash SSDs, such as the Intel X25-E (released 2010), have much higher IOPS than traditional HDD. In a test done by Xssist, using Iometer, 4 KB random transfers, 70/30 read/write ratio, queue depth 4, the IOPS delivered by the Intel X25-E 64GB G1 started around 10000 IOPs, and dropped sharply after 8 minutes to 4000 IOPS, and continued to decrease gradually for the next 42 minutes. IOPS vary between 3000 and 4000 from approximately 50 minutes and onwards, for the rest of the 8+ hours the test ran.[8] Even with the drop in random IOPS after the 50th minute, the X25-E still has much higher IOPS compared to traditional hard disk drives. Some SSDs, including the OCZ RevoDrive 3 x2 PCIe using the SandForce controller, have shown much higher sustained write performance that more closely matches the read speed.[9]
