Disk Response Time and IOMark: Average Positioning Speed

In this test IOMeter is sending a stream of requests to read and write 512-byte data blocks with a request queue of 1 for 10 minutes. The total number of requests processed by the HDD is over 60 thousand, so we get a sustained response time that doesn’t depend on the HDD’s buffer size.

Western Digital’s enterprise RE3 and desktop Caviar Black models feature the best read response time among all the drives. The result of lower than 12.5 milliseconds is very good for 7200rpm drives. Hitachi’s new models are just a third of a millisecond slower. They have improved by 0.5 milliseconds over their predecessor (but that’s not a record for this brand’s products because we have seen even better results from them). The 4-platter Seagate is also good with a response time of lower than 13 milliseconds.

What is the most interesting thing, the 3-platter WD Caviar Green with 32MB buffer has a response time comparable to that of the Samsung drives although its spindle rotation speed is only 5400rpm. The other 3-platter Green drive from WD is worse by 0.5 milliseconds, and the 4-platter models are worse by 1 millisecond. The 3-platter Seagate is quite a disappointment: 15 milliseconds is too much for a 7200rpm drive. Well, if the drive’s platters are moving slower, its noise is lower when processing a large number of random requests. But why did they stifle the drive like that? And the Seagate SV35.3 is even worse at 18.9 milliseconds!

We decided to check out this high value in more detail and see how that drive’s read access time was distributed – IOMark provides this opportunity. You should keep it in mind that we are talking about average values. The real access time depends on what exactly sectors the heads have to move between.

First, here is the access time distribution diagram for a similar product from a desktop series, the 4-platter Seagate Barracuda 7200.11.

So, everything is neat and fits within the standard distribution. You don’t have to be an expert in statistics to say that this HDD has an average response time of over 12 but less than 13 milliseconds. The top of the “hump” is exactly at that point.

Now let’s take a look at the same diagram built for the SV35.3:

The difference is notable indeed. There are few rather quick transitions (lucky situations when the previous and next sectors are in nearby cylinders) and a large number of very long seeks. This drive’s heads are moving very, very sluggishly. That’s a huge sacrifice for the sake of silence.

The write response results are interesting, too. We’ve got the same leaders: the two 7200rpm drives from WD and the Hitachi E7K1000. The 7K1000.B cannot keep up with the leaders, probably because of its 16MB buffer (the leaders are equipped with 32 megabytes of cache) that is not large enough for effective deferred writing. The comparison of the Green drives indicates this fact, too, because the model with 32MB buffer is faster. The difference between the 3-platter and 4-platter WD10EACS models is small because recording density has almost no effect on the response time (take note of the good result of the 5-platter Hitachi).

The three drives from Seagate produce shocking results. The SV35.3 has inefficient deferred writing that makes it far slower than the worst of WD’s Green models. Perhaps the difference would be small if this drive had quick heads, but the silence-oriented optimization affected its writing speed badly. The other two HDDs from Seagate have a higher response at writing than at reading. This might set us aback if we hadn’t seen the same thing in our earlier reviews. We don’t know if it is a problem of the electronics or firmware, but comparing the results of the 3- and 4-platter models it seems that Seagate has tried to solve the problem, but not quite successfully. Hopefully, the upcoming 7200.12 series will be free from this drawback.

We will do one more test in this section. It is similar to measuring the read response time but takes into account the number of platters and recording density.

So, it is the test of average positioning speed. The drive is being bombarded with read requests like in the response time test, and we calculate the difference between the LBA addresses of the previous and next requests and divide it by the time it took to perform the request. In other words, we have the distance (in gigabytes) the drive can run through in 1 second. The results are averaged and compared.

There are no surprises here. We’ve got the same leaders: the two 7200rpm drives from WD and two new Hitachi models. The old Hitachi 7K1000 is fifth thanks to its five platters. Then the standings repeat those of the response time diagram but Seagate’s drives take two last places. The 3-platter Seagate Barracuda 7200.11 cannot do anything even with its highest recording density: its slow heads kill its performance. The SV35.3 is only half as fast as the leader.