This is a very good explanation of why you need a battery backup unit if you do hardware RAID with write cache enabled
http://www.hostdime.com/blog/bbu-raid/
“What is a BBU for RAID and do I need one?” This question comes up often for HostDime technicians, so let’s examine the purpose of a BBU, which stands for “battery backup unit.” This awesome device protects and maintains the cached data that is on your server’s raid card. A BBU is essentially a data fail-safe.
Here’s a scary scenario: say for one reason or another, your dedicated server loses power before changes have left the cache and are committed to disk. You are now left with an non-completed query, or worse, corrupted data.
The battery backup unit allows the RAID card to remember what hasn’t yet been synced to disk. The BBU can provide enough backup power to preserve the data for up to 72 hours without power. When the machine powers back up, the BBU will write the cache contents on the disk. All contents on the RAID card buffer become unrecoverable if there is no BBU. The worst part is, you may not even know what files were damaged or corrupted.
If your “write cache” option is set to “write through” or “off”, then you should be fine without a BBU on your raid card. Many RAID cards will have the write cache setting “on” only if the BBU is installed. The downside to having “write cache” turned off is RAID performance will be sub-optimal. This is very important for users who frequently save their database from certain types of corruption or need high data integrity.