RAID Calculator

RAID 0 stripes with no parity, so usable = total raw. RAID 1 mirrors, so usable = one disk. RAID 5 burns one disk for parity, so usable = (N - 1) x disk size. RAID 6 burns two disks for double parity. RAID 10 mirrors, so usable = N/2 x disk size. RAID 50 and 60 stripe across multiple sub-arrays, losing more disks to parity.

RAID 0: zero (one failure loses everything). RAID 1: N - 1 (the array survives as long as one mirror remains). RAID 5: 1. RAID 6: 2. RAID 10: at least 1 and up to N/2 if failures land in different mirrors. RAID 50: one failure per sub-array. RAID 60: two failures per sub-array.

There is no reliable cutoff based only on drive count and capacity. RAID 5 rebuild risk depends on drive error rates, rebuild duration, workload, controller behavior, monitoring, and the chance of another failure. Model those factors for the array and keep independent backups. RAID 6 or RAID 10 may reduce some failure risks, but neither replaces a backup.

A small write may require reading old data and parity, then writing new data and parity. This is commonly called a four-I/O write penalty, but it does not mean throughput is always one quarter of read throughput. Cache, stripe width, queue depth, workload, controller design, and full-stripe writes all affect measured performance.

No. RAID protects against disk hardware failure but not against accidental deletion, ransomware, fire, theft, controller failure, or filesystem corruption. Use RAID for uptime and rebuild-without-restore, and use independent backups (3-2-1 rule: three copies, two media, one off-site) for actual data protection.