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RAID Calculator

What is RAID?

RAID (Redundant Array of Independent Disks) combines multiple physical drives into a single logical unit to improve performance, redundancy, or both. Different RAID levels offer varying trade-offs between capacity, speed, and fault tolerance. RAID 0 maximizes performance by striping data across drives but offers no redundancy. RAID 1 mirrors data for full redundancy at 50% capacity cost. RAID 5/6 use parity for efficient redundancy, while RAID 10/50/60 combine striping with mirroring or parity for enterprise-grade performance and protection.

Understanding RAID Levels

Each RAID level serves different needs. RAID 0 stripes data for maximum speed (2× IOPS with 2 drives) but one drive failure loses everything. RAID 1 mirrors data for safety but uses only 50% capacity. RAID 5 adds parity (minimum 3 drives) allowing 1 drive failure with ~67-93% efficiency. RAID 6 uses double parity (minimum 4 drives) surviving 2 failures. RAID 10 combines mirroring and striping for excellent performance with 50% capacity. RAID 50/60 stripe multiple RAID 5/6 groups for enterprise workloads requiring both speed and resilience.

Frequently Asked Questions

What is the difference between RAID 5 and RAID 6?

RAID 5 uses single parity and can survive 1 drive failure, requiring minimum 3 drives. RAID 6 uses double parity and can survive 2 drive failures, requiring minimum 4 drives. RAID 6 is safer for larger arrays (>8 drives) due to lower URE risk during rebuild, but has slightly lower write performance due to calculating two parity blocks instead of one.

Why is RAID 10 faster than RAID 5/6?

RAID 10 has a write penalty of only 2× (one write to each mirror), while RAID 5 has 4× penalty (read data, read parity, write data, write parity) and RAID 6 has 6× penalty (double parity). For read operations, both scale linearly, but RAID 10's lower write penalty makes it significantly faster for write-heavy workloads like databases and virtual machines.

What is URE risk and why does it matter?

URE (Unrecoverable Read Error) is when a drive cannot read a sector during rebuild. With large modern drives (>6TB), the probability of hitting a URE while rebuilding from a failed drive is significant. Consumer HDDs have URE rates of 1 in 10^14 bits (~12.5 TB), meaning a single URE can cause complete data loss in RAID 5. RAID 6's double parity protects against this - even if a URE occurs during rebuild, the second parity can recover the data.

How long does RAID rebuild take?

Rebuild time depends on drive capacity, array size, and rebuild speed. A 4TB drive rebuilding at 100 MB/s takes ~11 hours. An 18TB drive takes ~50 hours. During rebuild, array performance degrades significantly and URE risk is highest. RAID 6/60 is recommended for large arrays to reduce URE risk. Enterprise drives with lower URE rates (1 in 10^15) also help.

Can I expand a RAID array later?

It depends on your RAID controller and level. Some hardware controllers support online capacity expansion (adding drives) and RAID migration (changing levels). However, this is risky and slow - often taking days for large arrays. The safest approach is to plan array size upfront, backup data, destroy the old array, create a new larger one, and restore. Software RAID (mdadm, ZFS, unRAID) often has better expansion support than cheap hardware RAID.

Is RAID a replacement for backups?

NO. RAID protects against drive failure only - it does NOT protect against: file deletion, corruption, malware/ransomware, controller failure, fire/theft, or human error. The 3-2-1 backup rule applies: 3 copies of data (production + 2 backups), 2 different media types (e.g., RAID + external), 1 offsite copy (cloud or remote location). RAID is for uptime, not backup.