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Random Read & Write Patterns

Now we will see how the performance of the drives in random read and write modes depends on the size of the requested data block.

The random read results agree with the results of the response time test with a couple of curious exceptions. The 640GB WD BEVT slows down too much on very large data blocks while the Seagate 7200.4 has a sudden acceleration on 4 to 128KB data blocks, producing a very odd hump in the graph. It is the first time we ever see a graph like that and cannot think of an explanation for it.

We’ve got interesting results at random writing. The 500GB Western Digital and Hitachi 7K500 are the clear winners. The Hitachi 5K500.B behaves differently from its faster cousin and its performance with medium-sized data blocks is nothing more than satisfactory. Well, the Seagate 7200.4 performs even worse. It seems to have a strictly limited number of cache lines, which explains its low performance with data blocks of any size, save for large ones.

The WD drives with 4KB sectors behave just as expected. As long as the request is smaller than the size of one sector, their performance is low, being limited by the extra read and modify operations. But when the request is larger than one sector, these HDDs speed up and get closer to their predecessors. The 750GB model is ahead, which seems to be due to some firmware optimizations rather than to its higher recording density.

 
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