We tested only 4-HDD arrays in WriteThrough mode. The results are summed up in the table below:

To compare the performance of 4-HDD RAID arrays with different caching algorithms we will put together a traditional table. The number in each table cell is the ratio of the array speed in WriteBack mode to its speed in WriteThrough mode. If the result is less than 1 (these numbers are usually highlighted red, but this time there are none), then WB caching is not efficient here. If the result is bigger than 1(these numbers are highlighted blue), then WB-caching has had its positive effect on the performance. If the result equals 1.0, both – WB and WT caching are equally efficient in this work mode.

As we have expected, disabling WB-caching in the BIOS reduces the array performance in all work modes, however, it is for the first time that our result table doesn’t have a single red number in it. It means that WriteBack mode of 3ware 9500S-8 controller has only positive effect on its performance. However, the performance of RAID10 array with different controller cache settings in RandomRead mode is pretty strange: with 0% writes the performance of this array type should not depend on the caching algorithms.

Now let’s take a look at the graphs showing the arrays performance with different caching algorithms involved. For your convenience we will build the graphs for each array type in WriteThrough and WriteBack modes for queue of 1, 16 and 256 requests:

With the disabled controller cache the RAID0 array slows down a lot even when there are few write requests present. As the share of writes increases, the performance difference gets bigger, too reaching 7.5x. The more requests are in the queue, the lower is the performance difference between the WriteBack and WriteThrough modes. However, the advantages of WB-caching show themselves in all work modes except RandomRead, because there are no write requests and hence lazy writing algorithms do not affect the performance.