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 number of outstanding requests varying from 1 to 8. The clients’ address zones do not overlap. We’ll discuss diagrams for a request queue of 1 as the most illustrative ones. When the queue is 2 or more requests long, the speed doesn’t depend much on the number of applications.

• IOMeter: Multithreaded Read RAID1+RAID10
• IOMeter: Multithreaded Read RAID5+RAID6
• IOMeter: Multithreaded Write RAID1+RAID10
• IOMeter: Multithreaded Write RAID5+RAID6

We want to say a word in defense of the 8-disk arrays: they only show their top speeds at queue depths other than 1. We have seen this before in our tests of top-performance controllers with SAS disks, so we know that you should have a look at the table with the numbers. It should be noted, however, that the Adaptec delivers high speed even at the minimum queue depth. Its results are the best among all the controllers we have tested so far.

When reading one thread, the arrays are ranked up in a normal manner: the 8-disk arrays are in the lead, the RAID0 are ahead, followed by the RAID5 and RAID6. Interestingly, the 4-disk RAID10 is the fastest among the 4-disk arrays.

The standings change dramatically when there are two threads to be processed. The RAID10 have the smallest performance hit, even though without showing the ability to parallel the load by reading the threads from the different disks in the mirrors. The 8-disk arrays are all good, but the 4-disk ones have a difficult time. The degraded arrays, even the RAID6 without two disks, pass this test quite successfully, excepting the RAID10 which is as fast as the 4-disk RAID0.

When the number of read threads is increased to three and four, the RAID10 arrays lose their advantage, the 8-disk one having the biggest performance hit. The other arrays just slow down a little.

At one write thread we see those odd problems again. As a result, the degraded arrays cope best with writing. They are followed by the RAID10 whereas the 8-disk RAID5 and RAID6 and both RAID0 fall behind the single HDD even.

The addition of a second write thread brings about significant changes we cannot find a reasonable explanation for. For example, the degraded checksum-based arrays and all of the 4-disk arrays, excepting the RAID0, slow down (the 4-disk RAID6 and RAID10 have a considerable performance hit) whereas the other arrays speed up. The 8-disk RAID5 and RAID6 improve the speed twofold! And this is quite inexplicable because these arrays still have to process the same checksums with two write threads as with only one thread.

The number of write threads increases, and the performance of the 8-disk RAID5 and RAID6 falls below that of their 4-disk counterparts. As if to balance them off, the 4- and 8-disk RAID10 accelerate again. The degraded RAID10 differs from both the other degraded arrays and from the RAID10 and is about as slow as the 4-disk RAID0.