RAID (Redundant Array of Independent Disks) – This technology is mainly used for hard disks to provide protection against hard drive failures and also to improvise performance (of course you can select either of failure protection / performance). RAID also gives an implicit way to scale your disks (after all it’s an Array). RAID builds on 2 primary concepts – Striping (storing data across multiple disks sectors of multiple drives so that disk head can read more data in a single move) and Mirroring(replication of drives). RAID requires minimum of 2 disks & can have maximum of 32 disks. It’s also important to note here that RAID controllers typically abstract out the striping & mirroring details giving end user a feel of only a single disk available at disposal. Below are some of the popular RAID configurations out there:
RAID 0 – Striping
Pros: Better performance – data replicated across drives, no storage overhead as drives are utilized 100%
Cons: Possibility of losing entire data on failure of a single disk
Minimum Disks Required – 2
RAID 1 – Mirroring
Pros – Guard against disk failure as data is replicated across disk drives
Cons – Replication creates storage overhead as the same data is copied across drives
Minimum Disks Required – 2
RAID 4 – A Mirroring variation. It actually uses the concept of ‘Parity’ (based on XOR operation – Sum of all bits across drives – if sum is even parity set to 0 else parity set to 1) to retrive data in case of a disk failure, which reduces storage overhead – an issue with RAID 1.
Pros – Reduced storage overhead (actually we need only a single disk here to store parity). E.g. if you have 3 disks, parity can be stored on 3rd. So your overhead is only 33% in terms of storage.
Cons – Still suffers from a performance perspective
Minimum Disks Required – 3 (anyways 2 doesn’t make sense and the more no. of disks you have lesser would be your storage overhead)
RAID 5 – Striped Data & Striped Parity – most widely used. Taking RAID 4 to next level by striping the parity (parity gets stored across disks instead of getting stored on single disk) and also striping data (that anyways doesn’t matter as far as parity is concerned)
Pros – Good Performance, Good failure protection
Cons – Not as good when your requirement is only performance or only failure protection (parity doesn’t come for free).
Minimum Disks Required – 3
RAID 10 (1 + 0) – Combination of Stripe & Mirror with no parity. You stripe the data across drives & mirror the entire set of drives.
Pros: Best in terms of performance & guards against potential failures.
Cons: Costly in terms of storage overhead
Minimum Disks Required – 4 (and for > 4 you must have even number of disks)
