Random writing goes next.

• IOMeter: Random Write, operations per second
• IOMeter: Random Write, MBps

It is cache memory that determines the results of writing in small data chunks. The controller shows almost ideal operation: nice scalability, perfectly shaped graphs and high speeds (the speeds are higher than with the 3ware 9690SA controller we tested recently).

The number of disks in the array is still important for the checksum based arrays when they are writing data in small blocks, but the checksum calculations are a negative factor. The RAID6 have to process two checksums, therefore even the degraded RAID5 proves to be faster than the 8-disk RAID6. The degraded RAID6 without two disks feels surprisingly good here. Of course, it takes last place and is slower than the single HDD, but its performance is good enough for an array that has not only to read data but also restore them from two checksums in order to perform a write operation.

There is something fundamentally wrong in this test: the RAID10 are for some reason slower than the RAID10 when processing such data blocks. This is especially clear with the 8-disk RAID0 because this array has no cause for not being able to overcome the 120MBps mark. And the 4-disk RAID0 has no cause to be inferior to every other array, and even to the single HDD, at large data blocks.

The behavior of the degraded RAID10 is understandable, though. It behaves like the RAID0 array, i.e. each mirror behaves like a single disk. But why don’t the ordinary RAID10 behave like that? And what is going on with the arrays at large data blocks? Distributing the data blocks among the stripes of a RAID0 array is quite easy we might think, but there is some problem here.

Yes, the Adaptec controller has problems with writing in large data blocks indeed. The checksum-based arrays are almost as fast as the single disk, the 4-disk arrays being faster than the 8-disk ones. But each of the three degraded arrays has sped up suddenly, outperforming their healthy counterparts. We might explain this by the degraded arrays’ not having to process checksums that go to a failed disk, but the RAID6 without two disks would win then. And this wouldn’t provide such a high performance gain anyway. So, the controller behaves very oddly indeed when writing data in large blocks.