Raid and Reliability

Raid: redundant array of independent disks - standard way of organizing disks

Raid 1

Raid 2

Raid 3

Raid 4

$$$ parity = block1 \xor block2 \xor block3 parity' = block1' \xor block2 \xor block3

anything \xor anything = 0 anythin \xor 0 = anything parity \xor parity' parity' = block' \xor block1 xor parity $$$

Raid 5

• sort of the most general level of RAID
• spread out parity over all disks
• pros & cons
  • no parity disk bottleneck
  • all disks contribute read bandwidth
  • requires 4 I/Os for small writes
  • May not be good enough to protect devices >2TB (per storage device)

Raid 6

• use horizontal and diagonal parities
• pros & cons
  • can protect against double failures
  • additional overhead to compute and update additional parities
  • silent bit flip in one block can propagate