Erasure Coding Calculator
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How to Use This Calculator
Section titled “How to Use This Calculator”This calculator helps you understand the real trade-offs in erasure coding schemes. Input your data shards and parity shards to see actual storage efficiency, failure tolerance, and cost implications.
What the Numbers Mean
Section titled “What the Numbers Mean”Storage Efficiency: The percentage of raw capacity you can actually use for data. A 12+4 scheme uses 12 data shards out of 16 total, giving you 75% efficiency.
Storage Overhead: The extra capacity consumed by parity. Lower is better, but comes with trade-offs.
Failure Tolerance: How many simultaneous drive failures the system can survive. Standard Reed-Solomon delivers full theoretical tolerance. Local reconstruction codes (like VAST’s LDEC) reduce practical tolerance despite marketing claims.
Rebuild Efficiency: The percentage of remaining shards you must read to reconstruct a failed shard. Standard Reed-Solomon requires reading k data shards (typically 66-75% of total). Local reconstruction codes (LEC/LDEC) achieve faster rebuilds (~25%) by organizing shards into local groups, but this fragments parity protection.
Local Reconstruction Toggle: When enabled, simulates local erasure codes (LEC) and similar schemes. Since proprietary implementations like LDEC aren’t publicly documented, we model them as LEC with potential minor optimizations - the fundamental math can’t differ significantly due to information theory constraints. The calculator adjusts rebuild reads to ~25% and shows practical failure tolerance (typically 2 for wide stripes like 146+4) versus theoretical parity count. This reveals the core trade-off: faster rebuilds through locality come at the cost of fragmented parity and reduced practical protection.
Common Configurations
Section titled “Common Configurations”Try these standard industry configurations to see how they compare:
- 3x Replication: 2+1 (200% overhead, maximum simplicity)
- Standard Configuration A: 8+3 (37.5% overhead, 3-failure tolerance)
- Standard Configuration B: 12+4 (33.33% overhead, 4-failure tolerance)
- Standard Configuration C: 8+4 (50% overhead, 4-failure tolerance)
- Wide Stripe: 146+4 (2.74% overhead, theoretical 4-failure tolerance)
The Math Behind It
Section titled “The Math Behind It”Storage efficiency = data shards ÷ total shards Overhead = parity shards ÷ data shards × 100% Theoretical failure tolerance = number of parity shards Practical failure tolerance = depends on locality (standard RS delivers full tolerance)
Raw capacity needed = usable capacity ÷ storage efficiency
This calculator assumes standard Reed-Solomon erasure coding without locality optimizations. If a vendor claims “fast rebuilds” or “local reconstruction,” they’re likely trading failure tolerance for rebuild speed.
Why This Matters
Section titled “Why This Matters”Marketing focuses on low overhead percentages. The math reveals what you’re actually getting. A scheme with 2.74% overhead sounds amazing until you calculate that practical failure tolerance drops from 4 to 2 due to local group fragmentation.
Use this calculator to validate vendor claims. If the numbers don’t match what they’re telling you, ask for the actual configuration and run the math yourself.