Did a test on two DL380 G5 controllers to check out the difference between having a battery or not on a P400 SAS RAID controller. The results are remarkable, to say the least.
Both machines are fairly identical when looking at disk configuration: both have a P400 SAS controller with an equal amount of storage, but srv1 has 512MB with battery, srv2 has 256MB without battery.
I ran a dd test of a file, and this is the output:
srv1:~# dd if=/dev/zero of=./dump.dump bs=1k count=8192000 8192000+0 records in 8192000+0 records out 8388608000 bytes (8.4 GB) copied, 48.282 seconds, 174 MB/s
srv2:~# dd if=/dev/zero of=./dump.dump bs=1k count=5120000 5120000+0 records in 5120000+0 records out 5242880000 bytes (5.2 GB) copied, 121.61 seconds, 43.1 MB/s
There's an astounding difference of 130MB/s between the two servers.
Looking at the system management homepage, installed on each server, reveals:
There it is, srv2 doesn't use write caching, because there's no battery. srv1 has a battery (and a bit more cache), and therefore has the ability to assign plenty of it to write caching.