Replication is a proven and valuable mechanism to ensure verify data integrity, but the cost per GB of storage can become high as object sizes and cluster sizes grow. A complementary data protection strategy, erasure coding (EC), provides high data durability with a smaller footprint. Swarm manages EC and replication together to optimize cost-effectiveness, converting objects between them seamlessly and dynamically, based on the policies set.
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Swarm creates multiple sets of erasure segments for very large objects. The object breakdown into one or more erasure sets is transparent to external applications. A GET or HEAD of an erasure-coded object uses the same syntax as a replicated object.
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Swarm still reads the object as long as there are still k total segments (any combination of original data or parity) remaining in the cluster if a hard drive or a node containing an erasure segment fails. Protection against drive failure for the object is equal to the number of specified parity p segments.
The segments from a 5:2 (5 data segments with 2 parity segments for a total of 7 segments) or 8:2 (8 data and 2 parity segments for 10 total segments) erasure code are protected against the loss of any two nodes because they are distributed to different nodes. An erasure-coded object is immediately retrievable when accessed even if some segments are missing. Regenerating the missing erasure set segments is performed in a self-healing, cluster-initiated manner (similar to the recovery process for replicated objects) to protect against further drive loss. This process kicks off automatically when a missing volume is detected and automatically regenerates any missing segments.
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(total segments ÷ data segments ) × object size | = object footprint |
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((k+p) ÷ k) × GB | = total GB |
How footprint changes with different EC encoding (versus 3 reps)
1 GB object with 5:2 encoding: ((5 + 2) ÷ 5) × 1 GB = 1.4 GB (vs. 3 GB for replication)
3 GB object with 5:2 encoding: ((5 + 2) ÷ 5) × 3 GB = 4.2 GB (vs. 9 GB for replication)
3 GB object with 7:3 encoding: ((7 + 3) ÷ 7) × 3 GB = 4.3 GB (vs. 9 GB for replication)
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