Speed up MDADM RAID5 array using stripe_cache_size

edit /etc/rc.local
        add:
                echo 4096 > /sys/block/md0/md/stripe_cache_size

To make effect take place immediately:
echo 4096 > /sys/block/md0/md/stripe_cache_size

4096 was chosen after fairly extensive bonnie testing of various sizes from 256 to 8192 with and without NCQ enabled on the drives. Using this setting increased my write speeds by about 50%.

References:
NeilB's post: "You can possibly increase the speed somewhat by increasing the buffer space that is used, thus allowing larger reads followed by larger writes. This is done by increasing /sys/block/mdXX/md/stripe_cache_size"

Another one of NeilB's posts regarding this topic:
"Changing the stripe_cache_size will not risk causing corruption.
If you set it too low the reshape will stop progressing. You can then set it to a larger value and let it continue.
If you set it too high you risk tying up all of your system memory in the cache. In this case you system might enter a swap-storm and it might be rather hard to set it back to a lower value.
The amount of memory used per cache entry is about 4K times the number of device in the array."

NeilB is Neil Brown, the author of MDADM.