Performance in Intel IOMeter: Sequential Read and Write Patterns

Now let’s pass over to sequential reading and writing. The array receives a stream of read/write requests with a request queue depth of 4. Every minute the size of the data block changes, so we can see the dependence of the linear read/write speed on the size of the data block by the end of the test session. The sequential read results (the dependence of the controller data transfer rate on the data block size) are listed in the following table:

Now let’s take a look at the dependence graph:

Different arrays show maximum read speed with requests of different size. In other words, when the controller splits a large request into a few smaller ones processed by several hard disk drives at a time. That is why the scalability of the RAID 0 array performance depending on the number of the drivers in this array can be seen only in case of large requests. At the same time I would like to stress that the maximum performance of a RAID 0 array of four hard disk drives is exactly (!) four times bigger than the performance of a single HDD and equals 274MB/s. And this number is higher than the bandwidth of the 66MHz PCI bus (266MB/s). This is exactly the reason why Silicon Image used the 133MHz PCI-X bus for its SiI3124 controller.

Unfortunately, the situation with mirrored arrays is much worse. As we remember, some manufacturers alternate read requests between the drives of a mirrored pair, even if we have linear reading. This way, RAID 1 array performs like RAID 0 array on reading operations, which may (theoretically) double the read speed from the array. But as we see, the SiI3124 controller doesn’t apply this algorithm even for sequential reading. The read speed from RAID 1 array in all modes coincides with that from a single HDD, and the performance of RAID 10 array equals to that of 2-HDD RAID 0.

Now let’s take a look at sequential writing:

The graph below shows the dependence of the controller write speed on the size of the processed data block:

During sequential writing, just like during sequential reading, different arrays achieve their maximum speed at different request sizes. When the request size exceeds 32KB, the scalability of the RAID 0 performance depending on the number of the drives in an array is not just visible, but is close to ideal (which is a very rare thing for any controller working under such workload). Moreover, the maximum performance of RAID 0 array of four hard drives is exactly four times bigger than the single HDD speed, and equals 272MB/s, i.e. is the same as the read speed. In this working mode it is only for the better that SiI3124 controller has no cache, as it doesn’t affect its performance and stability.

The write speeds of RAID 1 and RAID 10 arrays are equal to the single HDD write speed and to the 2-HDD RAID 0 array write speed in all test modes. However, when there are no read requests at all, this behavior is typical of mirrored arrays.