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Random Read & Write Patterns

Now we’ll see the dependence between the drives’ performance in random read and write modes on the size of the data block.

We will discuss the results in two ways. For small-size data chunks we will draw graphs showing the dependence of the amount of operations per second on the data chunk size. For large chunks we will compare performance depending on data-transfer rate in megabytes per second. This approach helps us evaluate the disk subsystem’s performance in two typical scenarios: working with small data chunks is typical for databases. The amount of operations per second is more important than sheer speed then. Working with large data blocks is nearly the same as working with small files, and the traditional measurement of speed in megabytes per second becomes more relevant.

Let’s start with reading.

When reading small-size data blocks, the HDDs are ranked up according to their response time we have measured in the previous test. Thus, the old Maxtor Atlas 15KII is first while the NS.2 with slow platters is last.

When the HDDs are reading large data blocks, their sequential read speeds become the crucial factor and the standings depend on recording density. The newer the HDD, the higher its result is. A good response time can only make a difference here if the other factors are equal.

The graphs of writing small random-address data blocks makes us recall the response time test again. The HDDs that had a high response time are bad in this test, too. The 146GB Seagate 15K.6, 73GB Seagate 15K.5 and Hitachi 15K147 draw almost horizontal graphs whereas the other HDDs accelerate as the data block is getting smaller. This must be due to some flaws in the firmware of the specific HDD models.

It is the speed of sequential writing, which depends on recording density, that is the decisive factor when the HDDs are writing large data blocks. You can see that starting from 512KB blocks.

As for the Seagate NS.2, it is just a little slower than the previous-generation 15,000rpm products at three out of four loads. However, it is no match to its older but devilishly fast opponents at reading small random-address data.

 
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