The short answer: For small business servers, RAID 1 suits two drives, RAID 10 gives the best performance for databases and virtual machines, and RAID 6 gives the most usable capacity while surviving two drive failures. RAID 5 is falling out of favour with large drives. Whichever you choose, RAID keeps a server running through a drive failure; it is not a backup.
What RAID does
RAID combines several drives so that a server keeps working when one fails and, depending on the level, runs faster or offers more capacity. Most business servers use a hardware RAID controller, such as Dell PERC or HPE Smart Array, which manages the drives independently of the operating system.
RAID 1: mirroring
- Drives: 2
- Usable capacity: one drive's worth (50%)
- Survives: one drive failure
- Good for: the operating system, and small servers with modest storage
RAID 5: striping with single parity
- Drives: 3 or more
- Usable capacity: all the drives minus one
- Survives: one drive failure
- Watch out: after a failure the array rebuilds by reading every remaining drive. With today's large drives that takes a long time, and a second failure or a read error during the rebuild can lose the array. That's why many administrators now choose RAID 6 or RAID 10 instead.
RAID 6: striping with double parity
- Drives: 4 or more
- Usable capacity: all the drives minus two
- Survives: two drive failures
- Good for: file servers and backup targets with large drives, where capacity matters more than write speed
RAID 10: striped mirrors
- Drives: 4 or more, in pairs
- Usable capacity: half the total
- Survives: at least one drive failure, and more if the failed drives are in different pairs
- Good for: databases, virtual machines and anything with heavy random writes. Rebuilds are quick, because only the mirror partner is copied.
Which to choose
- Two drives: RAID 1.
- A virtualisation host or database server: RAID 10.
- A large file store or backup repository: RAID 6.
- Operating system plus separate data: RAID 1 for the operating system, and RAID 10 or RAID 6 for the data, on separate sets of drives.
Two things RAID won't do
RAID is not a backup. It protects against a drive failing, not against deleted files, ransomware, corruption or the loss of the whole server. Keep backups that follow the 3-2-1 rule: three copies of your data, on two different types of storage, with one copy off-site.
It won't warn anyone by itself. Configure a hot spare if you have a free drive bay, so a rebuild starts straight away, and make sure drive failure alerts reach someone who will act on them.
Need help?
If you're specifying a new server or storage array, request a quote and our team will recommend a layout for your workloads, or browse our storage solutions.
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