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Average Read/Write Response Time

The first test in this review is meant to check the average response time of a hard disk drive at reading/writing random sectors. The goal of the test is to measure the disk access time at reading and see how aggressive the drives’ deferred write policy is (roughly speaking, we are going to see how many segments are allocated in the drive’s cache for storing write requests).

So we make IOMeter send a stream of requests to read and write 512-byte data blocks with the request queue depth set at 1 for 10 minutes. The total number of requests processed by the drive is over 60 thousand. This ensures we get a sustained disk response time in the end, which doesn’t depend on the size of the drive’s cache buffer.

Here are the results of the drives in IOps (the number of requests processed per second). The more requests the drive manages to process, the faster the drive is, as you can easily guess.

The two WD740GD clearly use different read-ahead policies. More interesting is that they also take different approaches to processing write requests! The WD1500AHFD is in between them in reading speed, but is far faster than both the WD740GD at processing write requests. Well, it would have been strange if it had done otherwise having twice the amount of cache memory.

The results can be translated from IOps into traditional parameters – read and write response time.

The results themselves do not change, of course. They are just represented in a different way.

I’ll soon get back to examining the drives’ behavior at processing random read and write requests. Right now let’s check the speed of reading from and writing into the drive’s cache buffer.

 
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