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Multithreaded Read & Write Patterns

The multithreaded tests simulate a situation when there are one to four clients accessing the virtual disk at the same time – the clients’ address zones do not overlap. We’ll discuss diagrams for a request queue of 1 as the most illustrative ones.

Here is the first surprise, and a rather unpleasant one. Neither array can reach a read speed of 80MBps with one thread, which is a very low result especially for RAID0 arrays. The read speed of the single disk is 63MBps, so the performance gain is very small. You can also note another oddity here: the RAID0 and RAID5 arrays with many disks are slower than the same-type arrays with fewer disks.

When the number of threads is increased, the mirrored arrays go ahead. The speed of the two-disk RAID1 indicates that the data are being read at full speed from both disks of the mirror. The RAID10 delivers an even higher speed but without the correlation the RAID1 shows. The other array types are still very slow – and slower than at reading one thread. The standings have become more logical, though. The RAID5 are faster than the RAID0 and inside each of these array types the speed grows up depending on the number of disks in the array.

Of course, these low results may be also due to the HDDs’ algorithms that are allowed to work in the controller’s Performance mode. The Areca 1220 controller had better results in this test, however.

There are no surprises at writing. The two-disk RAID1 is as fast as the single disk. The two-disk RAID0, three-disk RAID5 (one of its disks stores the checksum rather than data even though the checksum is evenly distributed among the disks in practice) and four-disk RAID10 are two times faster. The three-disk RAID0 and four-disk RAID5 are three times as fast as the single disk. The four-disk RAID0 is four times as fast as the latter.

The standings do not change at two threads – the arrays just slow down somewhat. All the arrays, save for RAID5, survive the addition of more threads well enough. They only lose a little more speed. The RAID5 type slows down to the level of the single disk at three and four threads. This must be the processor’s fault.

 
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