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.