Sequential Read & Write Patterns

IOMeter is sending a stream of read and write requests with a request queue depth of 4. The size of the requested data block is changed each minute, so that we could see the dependence of an array’s sequential read/write speed on the size of the data block. This test is indicative of the highest speed a disk array can achieve.

• IOMeter: Sequential Read
• IOMeter: Sequential Write

The 8-disk RAID0 delivers an astonishing performance! The controller’s fast processor is strong enough for reading over 100MBps from each of the eight disks. The controller obviously reads from both disks in the mirrors, although not ideally: the 4-disk RAID10 is about as fast as the RAID0 while the 8-disk RAID10 is slower than its mirror-less counterpart. It is good that reading from both disks in mirrors works even on very small data blocks.

The checksum-based arrays have lower speeds than the RAID0, but that’s logical: one and two blocks are lost in each stripe with RAID5 and RAID6, respectively, due to the writing of checksums. As a result, the read speed degrades when all of the disks are being accessed. It is clear how dear in terms of performance the loss of disks is for the degraded arrays. Besides the reduction of speed due to the decreased size of the array, the degraded arrays lose about 200MBps more. Thus, the cost of data recovery equals the loss of two more disks when the degraded array is reading data. It is interesting that the RAID6 is almost indifferent to the loss of a second disk. The performance hit is only 100MBps, which is quite proportional to the reduction of the array size by one disk.

Well, the controller is not good at all at sequential writing. The performance of the RAID0 arrays is especially poor. They are slower than the single HDD! The degraded RAID10 is as worse as the RAID0: its graph almost coincides with the 4-disk RAID0’s (this explains the poor results in the test of writing large random-address data blocks). The controller might be suspected in having disabled deferred writing, but the full RAID10 arrays deliver normal speeds even though their graphs are not perfect, either. The zigzagging graphs indicate flaws in the controller’s firmware or driver.

The RAID5 and RAID6 arrays are not quite good at writing, either. The 8-disk arrays are inferior to the single HDD whereas their 4-disk counterparts are quite faster, even though do not reach the level that might have been expected from the performance of the single HDD. The degraded arrays deliver excellent performance, the RAID6 without two disks being in the lead. And the graphs are zigzagging again.

We hope these problems occur only when the controller meets the specific synthetic load and that we will see normal results in FC-Test.