Random Read & Write Patterns

Now we’ll see the dependence of the arrays’ performance in random read and write modes on the data chunk size.

We will discuss the results of the disk subsystems at processing random-address data in two variants basing on our updated methodology. For small-size data chunks we will draw graphs showing the dependence of the amount of operations per second on the data chunk size. For large chunks we will compare performance depending on data-transfer rate in megabytes per second. This approach helps us evaluate the disk subsystem’s performance in two typical scenarios: working with small data chunks is typical of databases. The amount of operations per second is more important than sheer speed then. Working with large data blocks is nearly the same as working with small files, and the traditional measurement of speed in megabytes per second is more relevant for such load.

We will start out with reading.

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

The load is too low for the RAID arrays to show their best at simple random reading. Every array is slower than the single disk. The degraded RAID10 is surprisingly good especially as the other RAID10 arrays do not differ much from the RAID0 (we might expect them to be faster by means of reading from the mirrors).

It is similar with the checksum-based arrays: they are all somewhat slower than the single disk, the 4-disk arrays being inferior to the 8-disk ones. The RAID6 degraded by two disks is the worst array again, yet its performance is not really too bad.

When the arrays are processing large data blocks, the sequential speed becomes the decisive factor and the RAID0 arrays go ahead. Well, the 8-disk RAID10 is better than the 4-disk RAID0 which in its turn is about as fast as the 4-disk RAID10: the controller seems to be able to read data from both disks in the mirrors. Even the degraded RAID10 is ahead of the 4-disk RAID0. The loss of a disk in one mirror does not prevent it from reading from both disks in the remaining mirrors.

It is due to the higher sequential read speed that the RAID5 are faster than the RAID6. The arrays of the latter type are rather too slow, though. The performance slump of the 4-disk RAID5 at very large data blocks is no good: 200MBps should not be the limit for this array.

The degraded arrays behave predictably: the RAID5 and RAID6 slow down without one disk. The RAID6 with two failed disks is comparable to the single HDD.