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RAID

Overview​

RAID (Redundant Array of Independent/Inexpensive Disks) is a method of combining multiple physical disks into a logical unit to improve performance, redundancy, or both.

RAID 0 – Striping​

Data is split into blocks and written across all disks in parallel.

  • Minimum disks: 2
  • Performance: Highest read/write speed (scales with disk count)
  • Redundancy: None, one disk failure leads to loss of all data
  • Usable capacity: 100%

Best for: temporary data, caches, or other scenarios where speed matters more than reliability.

Pros:

  • Maximum read and write performance
  • Full storage capacity is usable
  • Simple to set up

Cons:

  • No fault tolerance whatsoever
  • One disk failure destroys all data

RAID 1 – Mirroring​

Data is written identically to all disks.

  • Minimum disks: 2
  • Performance: Faster reads (can read from either disk), same write speed as a single disk
  • Redundancy: Can survive failure of all but one disk
  • Usable capacity: 50%

Best for: OS drives or critical data where reliability is the priority.

Pros:

  • High redundancy, simple recovery
  • Fast read performance
  • Easy to understand and manage

Cons:

  • 50% of storage capacity is lost to mirroring
  • Writes are no faster than a single disk

RAID 5 – Striping with Distributed Parity​

Data and parity information are striped across all disks. Parity allows recovery of data if one disk fails.

  • Minimum disks: 3
  • Performance: Good read speed; write speed reduced due to parity calculation
  • Redundancy: Tolerates 1 disk failure
  • Usable capacity: (n - 1) / n (e.g. 3 disks → 66%)

Best for: general-purpose file servers balancing capacity, performance, and redundancy.

Pros:

  • Good balance of capacity, performance, and redundancy
  • Only one disk worth of capacity lost to parity

Cons:

  • Write performance penalty due to parity calculation
  • Rebuild times can be very long on large drives
  • Array is vulnerable to a second failure during rebuild

RAID 6 – Striping with Double Parity​

Like RAID 5, but with two independent parity blocks, allowing two simultaneous disk failures.

  • Minimum disks: 4
  • Performance: Slightly slower writes than RAID 5 due to double parity
  • Redundancy: Tolerates 2 disk failures
  • Usable capacity: (n - 2) / n (e.g. 4 disks → 50%)

Best for: large arrays or environments where rebuild time increases failure risk.

Pros:

  • Survives two simultaneous disk failures
  • Safer for large arrays where rebuild takes days

Cons:

  • Higher write penalty than RAID 5
  • Two disks worth of capacity lost to parity
  • Requires at least 4 disks

RAID 10 – Striping + Mirroring​

Combines RAID 1 (mirroring) and RAID 0 (striping): data is mirrored in pairs, then striped across pairs.

  • Minimum disks: 4
  • Performance: High read and write speed
  • Redundancy: Tolerates 1 failure per mirrored pair
  • Usable capacity: 50%

Best for: databases and high-throughput workloads that need both speed and redundancy.

Pros:

  • Excellent read and write performance
  • Fast rebuild compared to parity-based RAID levels
  • Simple recovery process

Cons:

  • 50% of storage capacity is lost to mirroring
  • Requires at least 4 disks, costs scale quickly

Comparison​

LevelMin. DisksFault ToleranceUsable CapacityPerformance
RAID 020 disks100%Very high reads & writes
RAID 12n-1 disks50%Fast reads, slow writes
RAID 531 disk(n-1)/nFast reads, slow writes
RAID 642 disks(n-2)/nFast reads, slower writes
RAID 1041 per pair50%Very high reads & writes

Important Notes​

  • RAID is not a backup, it protects against disk failure, not accidental deletion, corruption, or disasters
  • Rebuild time on large drives can take hours to days, during which the array is vulnerable
  • Hardware RAID controllers offer better performance and cache, but add cost and vendor lock-in
  • Software RAID (e.g. Linux mdadm, Windows Storage Spaces, ZFS) is a cost-effective alternative