Principles and computing method of Erasure Code for OceanStor 9000

Erasure Code supports two data protection levels: N+M and N+M:B.
N+M includes N+1, N+2, N+3, and N+4, and is suitable for scenarios with a large number of nodes.
N+M:B includes N+2:1 and N+3:1, and is suitable for scenarios with a small number of nodes.
N+M indicates that each data stripe contains N original data strips and M redundant data strips, ensuring high data integrity even when M nodes fail.
N+M:B indicates that each data stripe contains N original data strips and M redundant data strips, ensuring high data integrity even when M disks or B nodes fail.
Users only need to specify +M or +M:B for directories or files. OceanStor 9000 automatically selects the most appropriate N value based on the number of nodes. N ranges from 2 to 16, and M from 1 to 4.
To configure the redundancy ratio:
1. Log in to DeviceManager.
2. Choose Provisioning > Resource Manager.
3. On the page that is displayed, select a directory and click Properties.
4. In the dialog box that is displayed, click the Redundancy Ratio tab and set +RDC/RDN.
In N+M scenarios, data slices are stored on N nodes, and redundant data is stored on another M nodes. Among the N+M nodes, up to three are allowed to fail at the same time. However, N is within 2 to 16 and OceanStor 9000 supports a maximum of 288 nodes. Accordingly, some nodes (except the N+M nodes) do not store data slices, and the failure of these nodes, in theory, does not impact on data integrity.
In N+M:B scenarios, data slices are stored on some disks of N nodes, and M pieces of redundant data are stored on other disks of the N nodes. The system allows up to M pieces of redundant data to be lost. That is, M disks are allowed to fail at the same time. Similarly, if more than B nodes fail, more than M pieces of redundant data will be lost. Therefore, a maximum of M disks or B nodes are allowed to fail.

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