Here some quick test results from a recently purchased Samsung SSD 840 Pro 256GB. The used benchmark software was ATTO v2.47.
The first test was run with a queue depth of 10 and total length of 2GB
In the second test smaller queue depth of 2 and length of 128MB were chosen. The results were unambiguously worse. The overall performance sank, the transfer rate was not continuously increasing, and the highest rates were achieved only under 4MB and 8MB transfer sizes, whereas similar results were observed on the first test starting with 32KB packets upwards.
Also interesting to observe was the disk utilization during the benchmark routine. You can spot the drop-downs between the 100% disk activity-peaks that correspond to the ‘breather’ between read and write cycles, which virtually invites for more parallel I/Os. But on the other side don’t ignore the read/write peaks during the 100% disk activity: these are the real limits of the SSD disk.