RAID: A Quick Summary
We provided detailed explanations of the common RAID modes 0, 1 and 0+1, in Part 1 of our RAID Scaling Charts. Similarly, you will find information on RAID 5 and RAID 6 in Part 2 of the RAID Scaling Charts. The RAID modes differ primarily in how they work with multiple hard drives, and hence have different characteristics. RAID 0 is ideal for maximum performance, but data has to be backed up on a regular basis to avoid data loss in case of a drive failure. RAID 1 is ideal to securely store data, because data is mirrored onto another drive. RAID 5 requires a serious RAID controller and runs with three or more drives. The array will still be functional if a drive fails, but performance isn’t as good as it is in RAID 0. RAID 6 creates double redundancy and hence keeps you data safe even if two drives fail, but to do this, RAID 6 needs at least four hard drives.
RAID does not have to be set up with physical hard drives, but it is often possible to create RAID arrays out of other arrays or storage partitions such as JBOD, where several drives are concatenated. Cascaded RAID arrays such as RAID 0+1 are referred to as Nested RAID. Popular options are RAID 5+0, where two or more RAID 5 arrays are put into a RAID 0 on a higher level. Using RAID 5 or RAID 6 arrays, which are more secure than individual drives, reduces the risk of losing data in a RAID 0.
Most people will recommend creating RAID arrays out of identical hard drives. It is in fact highly important to at least use drives with identical capacities, because the smallest capacity represents the lowest common denominator for the array: 4x 500 GB in a RAID 5 result in 1,500 GB net capacity (remember that one drive is used for parity data). However, using 2x 500 GB and 2x 320 GB will only give you the capacity of 4x 320 GB - the remaining storage capacity on the two 500 GB drive remains unused. Storage experts will even recommend using identical hard drives, which means that the drives’ firmware should be the same. Again, the weakest link, meaning the slowest drive, will determine performance.
Powerful RAID controllers always come with an integrated cache memory, or with a memory slot allowing the user to install an individual amount of memory. Caches work the same way as the cache you find on hard drives, but their much larger capacities ensure that their impact is higher. We used a powerful SAS RAID controller when we reviewed the HyperDrive 4 by HyperOS / Accelerated Logic. If you look at the data transfer diagram you’ll notice that this RAM-based solid state drive can write almost 90 MB/s. However, the first hundred megabytes can be written at an amazing speed of 450 MB/s, just because the RAID controller’s cache memory will buffer the incoming data. This effect can also be very positive for RAID arrays, but you should make sure that your RAID controller is equipped with a battery backup unit (BBU), or that the system is backed up by an uninterruptible power supply (UPS) to make sure that the cached data doesn’t get lost in case of a power outage.