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Depending on the erasure coding scheme that was selected during storage pool creation, erasure coded data is protected against failures as follows.
ECS requires a minimum of four nodes to be able to conduct erasure coding using the default erasure coding scheme. Erasure coding stops if a storage pool contains fewer than four nodes, meaning that the protection level is triple mirroring. During this time, the three replica copies remain, and parity is not calculated on any chunks. Once additional nodes are added to the storage pool and meet the minimum supported number of nodes, erasure coding continues on these nodes and new chunks.
For each 128 MB chunk, the default erasure coding scheme writes twelve data fragments and four coding fragments, each ~10.67 MB in size. It protects the chunk data against the loss of up to four fragments of a chunk, which can include the failure scenarios shown in the following table.
Number of chunk fragments per node | Erasure coded data protected against | |
5 nodes | 4 |
|
6 or 7 nodes | 3 |
|
8 or more nodes | 2 |
|
16 or more nodes | 1 |
|
Note: This table reflects protection levels possible with full distribution of chunk fragments. There might be scenarios where more fragments exist on a node, such as if a node has insufficient space available. In such case, the protection levels might vary.
ECS requires a minimum of six nodes to be able to conduct erasure coding using the cold storage erasure coding scheme. Erasure coding stops if a storage pool contains fewer than six nodes, meaning that the three replica copies remain and parity is not calculated on a chunk. Once additional nodes are added to the storage pool, erasure coding continues on these nodes and new chunks.
For each 128 MB chunk, the cold storage erasure coding scheme writes ten data fragments and two coding fragments, each ~12.8 MB in size. It protects the chunk data against the loss of up to two fragments of a chunk, which can include the failure scenarios shown in the following table.
Number of nodes in VDC | Number of chunk fragments per node | Erasure coded data protected against |
11 or fewer nodes | 2 |
|
12 or more nodes | 1 |
|
Note: This table reflects protection levels possible with full distribution of chunk fragments. There might be scenarios where more fragments exist on a node, such as if a node has insufficient space available. In such case, the protection levels might vary.