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Random Read & Write Patterns

Now we will 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 basing on data-transfer rate in megabytes per second.

Random-address reading produced expected results both with small and large data blocks. There is no need for detailed comments. Each drive’s performance with small data blocks depends on its response time whereas at data blocks of 2 megabytes and larger the sequential read speed becomes the crucial factor.

It is the drive’s response time that largely determines its performance with small data blocks in this random-address writing test but there are a few things we want to discuss in some detail. Take note of the Hitachi pair: they feel more confident with the smallest chunks of data, but their advantage over the Samsung and Seagate Barracuda XT shrinks as the data block size increases. The E3 model stands out among Western Digital’s products. As opposed to the similar FAEX (which still gives us no sign that it has a larger cache, by the way), the E3 is almost as fast on large data blocks as the previous-generation models.

The Green drive from WD is slow when unaligned because it has to do some extra operations due to its 4KB sectors. When aligned, it performs quite normally, its performance matching that of the Seagate Barracuda XT on large data blocks. It only has some problems with the smallest chunks of data where the alignment does not work because the request size is smaller than 4 kilobytes.

The alignment helps the WD Caviar Green on very large data blocks, too. When aligned, it is roughly as fast as the power-efficient models. WD’s Caviar Black products make it clear that HDDs with higher recording density per platter only show their advantage on data blocks larger than 2 megabytes.

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