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Random Read & Write Patterns

Now we’ll see the dependence of the disk subsystems’ performance in random read and write modes on the data chunk size.

We will discuss the results of the disk subsystems at processing random-address data in two variants basing on our updated methodology. 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 of 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 is more relevant for such load.

We will start out with reading.

The efficient reading from mirror pairs shows up in the test of reading in small data chunks: this is the reason why the controller can yield up to 200 operations per second on the RAID10. Even the degraded RAID10 has acceptable performance.

There is confusion among the checksum based arrays: the 4-disk RAID5 speeds up on 2KB data blocks while the degraded RAID6 (with two failed disks) suffers a performance hit on 8KB data blocks. Take note that the 8-disk arrays enjoy a larger advantage over the others as the data chunks grow bigger.

The speed of sequential reading becomes an important factor when the arrays are processing large data blocks. And we’ve got some odd results here. The 4-disk RAID0 can’t leave the single HDD behind whatever the data block size. The 8-disk RAID0 is no different from the other arrays even on 2MB data blocks although it should be faster then.

The healthy RAID5 and RAID6 are all right in terms of speed: their graphs are a treat for our eyes. But the degraded arrays are only as fast as the single HDD – we want more!

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