Erasure Coding

Erasure coding is what makes data on Sia durable. Instead of storing one copy of your data on one host, the SDK splits each slab into N data shards and adds M parity shards computed from the data. The original slab can be reconstructed from any N of the N+M shards — the rest can disappear and the data is still intact.

Sia's default is 10 data + 20 parity = 30 total shards per slab. Each shard goes to a different storage provider, so for a slab to become unrecoverable, more than 20 of the 30 hosts holding it would have to be unreachable simultaneously.

Why this matters

A naive way to make data durable is replication: store three copies, lose any two hosts and you still have one. That costs 3× the raw size and tolerates 2 host failures.

Sia's 10-of-30 erasure coding costs the same 3× — but tolerates 20 host failures instead of 2. The probability of losing data isn't just better; it's many orders of magnitude better.

Scheme	Storage overhead	Hosts that can fail
3× replication	3×	2
10-of-30 erasure	3×	20

For independent host failures at any reasonable rate, this is the difference between "data eventually disappears" and "data effectively never disappears."

Cost on the wire

Erasure coding isn't free on upload. Pushing a slab to the network transfers (N+M)/N × the raw size (3× by default). Downloads, on the other hand, only fetch the N data shards under normal conditions; parity is requested only if a data shard is unreachable, so download bandwidth tracks the raw size.

This is why progress for uploads vs. downloads uses different denominators.

Tunable durability

UploadOptions exposes data_shards and parity_shards. Higher parity gives more durability and more upload bandwidth; lower parity is cheaper and leaves a thinner margin. The SDK validates the combination, rejecting configurations whose recovery probability falls below a safe threshold or whose redundancy ratio is outside [1.5×, 4×], so you can't accidentally choose a setting that compromises durability.

The defaults are tuned for the typical case -- they do not need to be changed.