Now let’s take a look at the WebServer pattern, which is remarkable for having no writes at all. First come a few tables:
And now a few graphs:
As we see, the performance difference between the WT and WB modes is very small, even smaller than in FileServer pattern. However, there is one more general difference, which you can clearly notice from the graphs. In WebServer the graphs for a single hard disk drive and RAID 0 arrays start from the same point. The reason is very simple: under linear workload RAID 0 arrays lose their advantage of parallel HDD functioning. The arriving request is simply sent to the corresponding drive and each HDD works separately (if the requested data block is smaller than the stripe).
The absence of write requests allows taking full advantage of the mirrored arrays, and of course, they can’t miss this great opportunity. RAID 01 array is at the leading edge, while RAID 1 outperforms RAID 0 of two drives.
The rating system where we compare the performance of various RAID arrays (we calculate the rating the same way we did it for FileServer) shows the same picture as the graphs above. WB-caching results into an evident performance drop. The reason is also very simple: no write requests. Caching doesn’t have any positive effect in this case, while the driver still does use processor time. So, the overall performance decreases.