The Cached IO policy in LSI 2108/2208 controller IO policy only helps when IO has locality (e.g. There are IO hitting the same block multiple times). This is also associated with controller read ahead policy being set to caching ahead. The algorithm in Toshiba's SAS drives is dynamic cache, which always reads the bigger block size of data in the cache buffer. When the size of cache in the controller and the drive do not mach, the cache will need to be flushed out and reload the data. This is why the sequential read performance drops.

It is recommended then, to set the IO policy in the LSI controller to use direct IO and the read policy to be either normal, ahead, or adaptive. This way the sequential read performance drop can be avoided.