And now let’s have a look at another big table: the controller performance in WriteBack mode:
Now the graphs come:
Even under linear workload RAID 1 array is faster than a single drive, and RAID 01 array outperforms RAID 0 of two drives. As we remember, in WT mode we didn’t see anything like that. It means that WriteBack mode (WB, write requests caching performed by the driver) allows increasing the performance of mirrored arrays even under linear workload.
In case of 16 requests workload the advantages of mirrored arrays (RAID 1 and RAID 01) over the JBOD and RAID 0 of two drives respectively appear even more evident. RAID 0 arrays run pretty slow if the share of writes is low enough, just like in case of WriteThrough. However, in this case the performance drop is much greater. Look how aliased the graph for RAID 0 is! You can notice that for arrays of two, three or four drives the performance drop disappears when we hit 20%, 30% and 40% writes share respectively. By the way, as you remember, we saw this stepping effect in WT mode as well, that is why it is not the WB algorithms that produced it here. Since we have never seen an effect like that when testing any other SerialATA RAID controllers, I dare suppose that it is Promise controller drivers that tell here.