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 of the disk subsystems at processing random-address data in two versions. 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.
Reading small random-address data chunks goes on according to the results of the HDDs in the response time test above. The lower the response time, the higher the speed is. There are no inexplicable slumps or anything. Just a perfect picture.
It is the sequential read speed that affects the processing of large data blocks. Therefore the Seagate 7200.4 is ahead here whereas the Hitachi 5K500 slows down. The Fujitsu is the best among the 5400rpm drives based on 250GB platters.
The HDDs differ more when writing random-address small-size data blocks. The Western Digital is in the lead, followed by the Fujitsu. Third place goes to the Seagate 5400.6. The 7200rpm Seagate seems to have some problems with deferred writing as its speed is very low. The Hitachi 5K500 seems to have problems as well – that’s not right for a HDD to slow down like that.
It is the sequential write speed that becomes important when the HDDs are processing large data chunks and the Seagate 7200.4 is rising steadily from last to first place. The Hitachi 5K500.B is surprisingly poor. Although faster than its 3-platter predecessor, it is much slower than the other HDDs of its class.