Combining drives.
WHAT IT DOES
Combines several drives for capacity, performance, resilience, or a combination.
WHAT THE COMMON ARRANGEMENTS PROVIDE
- Striping: performance and capacity, no resilience
- Mirroring: a complete copy, surviving one failure
- Striping with parity: capacity with resilience to one failure
- Striping with double parity: resilience to two failures
Combinations of mirroring and striping
WHAT TO USE FOR PERFORMANCE WITH RESILIENCE
Mirrored pairs, striped.
WHAT TO USE FOR CAPACITY WITH RESILIENCE
Parity arrangements, accepting slower writes.
WHAT THE WRITE PENALTY IS
Parity arrangements must read and recalculate on every write.
WHAT THAT MEANS
They are poor for write-heavy workloads.
WHAT REBUILD IS
Reconstructing a replaced drive from the others.
WHY IT IS THE DANGEROUS PERIOD
Every remaining drive is read fully, under load, and a second failure loses everything.
WHAT MAKES THAT LIKELY
Drives from the same batch, of similar age.
WHAT THAT ARGUES FOR
Double parity on large arrays, and prompt replacement.
WHAT RAID IS NOT
A backup.
WHY
It protects against drive failure, not deletion, corruption, ransomware or disaster.
