In addition to spreading segments out across nodes (so no two segments for the same erasure set reside on the same node), Swarm automatically distributes segments evenly across subclusters. If it is configured correctly, this distribution keeps your data is protected even if an entire site fails.
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(Remaining subclusters) / (Total subclusters) >= k data-segments / (k data-segments + p parity-segments) |
Example: Subcluster setting that does not
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guarantee full recovery
With two subclusters containing four nodes each and EC set to 5:2 for seven total segments, one subcluster with three segments and the other with four segments. The number of segments in each subcluster is greater than the parity count (in this example), so the object is NOT fully recoverable if an entire subcluster is lost. Using the formula above to double check shows that the data is NOT safe if a single subcluster is lost:
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1/2 IS NOT >= 5 /(5+2) |
Example: Subcluster setting that does
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guarantee full recovery
With 3 subclusters containing 5+ nodes each and EC set to 9:6 for a total of 15 segments, each subcluster has five segments. If an entire subcluster is lost, five segments are lost, which is less than the set parity level, so the object is recoverable.
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The following table compares these examples.
EC Encoding | Nodes needed to write an EC object | Max nodes that can fail and still read EC object | Protected with loss | Protected with loss |
|---|---|---|---|---|
5:2 | 7 | 2 | NO. | NO. |
9:6 | 15 | 6 | NO. | YES. |
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