Given my present job role, I am often confronted with some assumptions people have about RAID arrays.
Thinking of those conversations, I decided to write a small and human-readable (I believe) article about this.
Let’s start by explaining what Hardware RAID is.
A RAID array, based in hardware requires a controller (AKA RAID Controler) physically installed in the system. Depending on your goal, and available hard drives, you can opt for several RAID levels—Please check about them here.
For not, let’s focus on what RAID can and cannot do for you.
What RAID will not be able to assure you?
Uptime of 100%. While a RAID based logical volume can offer some resilience and prevent some downtime, like many other systems, this is not a magical solution to have zero downtime issues. With RAID, the risk associated with hardware still exists. If a RAID Controler fails, you will have downtime to replace it. However, failures on such components are indeed less current than for example a hard drive failing, given its moving parts.
RAID is NOT a backup solution!
A frequently tested and properly planned backup solution will never be replaceable with a RAID array of any sorts.
You can suffer from data corruption, security incidents (ramson ware attacks for example), a myriad of human errors, and only a proper backup solution will rescue you by restoring a good set of data to replace the one you lost.
RAID will only protect you against hard drives fail on a running array an give you margin to replace the faulty drives (up to a limited number) without losing data or having downtime, but only this. I know I am repeating myself, but RAID is NOT a backup solution.
I would even advise that before considering RAID-based hardware, you should think ahead of a backup solution if you still don’t have one.
RAID does not always allow dynamically increasing in size the existing array.
If you need more space, in most cases, you cannot only add another drive to the array. Most of the times, it is true that you are going to have to start from the beginning, rebuilding and reinitializing the array.
Is RAID a good option for virtualisation and high-availability scenarios?
The answer is a straightforward No.
For such environments the right choice is based in SAN storage.