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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 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.

Like other RAID controller of its class with these HDDs, the LSI 8708EM2 produces its top speed only at request queue depths longer than 1. When there is one read thread, the 8-disk arrays show the best performance. The only exception is the 8-disk RAID10. As usual, the picture changes dramatically when we add a second thread. The same trio of 8-disk arrays still have high speeds but the 4-disk RAID0 has sped up twofold and now competes with the leaders. Interestingly, it has left behind the 8-disk RAID0, which has suffered a twofold performance hit. So, can the controller process two threads in parallel in mirror pairs? Perhaps it can but does not do that always.

The degraded RAID5 and RAID6 survive the addition of a second thread nicely. They even increase their speed. It looks like the second thread provides an opportunity for them to increase the queue and optimize their operation.

The 8-disk RAID10 wakes up at three threads and outperforms its 4-disk counterpart. The degraded arrays are still very good while the 8-disk RAID0 takes first place: the additional threads make up for the short queue depth for it, so it does not slow down at all.

Save for the RAID10, all of the 8-disk arrays, including the degraded ones, accelerate at four threads. This controller obviously likes to work with high loads and many disks. The RAID10 arrays lose their quickness, finding it hard to handle four read threads simultaneously.

The controller gives us yet another portion of oddities at writing: the degraded arrays are in the lead when processing one write thread. The 8-disk RAID0 is the slowest of all.

The second write thread provokes changes in the standings, but we don’t see any logic in them. Some arrays get faster and some slower, irrespective of the number of disks or array type.

When we add more write threads, the picture becomes somewhat clearer. The RAID5 and RAID6 arrays consisting of a large number of disks cope best with multithreaded writing. The RAID10 do not like this load and are even slower than the single HDD.

 
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