PowerScale OneFS 9.2.1.0 CLI Administration Guide

N+M data protection

OneFS uses data redundancy across the entire cluster to prevent data loss resulting from drive or node failures. Protection is built into the file system structure and can be applied down to the level of individual files.

Protection in OneFS is modeled on the Reed-Solomon algorithm, which uses forward error correction (FEC). Using FEC, OneFS allocates data in 128KB chunks. For each N data chunk, OneFS writes M protection, or parity, chunks. Each N+M chunk, referred to as a protection group, is written on an independent disk in an independent node. This process is referred to as data striping. By striping data across the entire cluster, OneFS is able to recover files in cases where drives or nodes fail.

In OneFS, the concepts of protection policy and protection level are different. The protection policy is the protection setting that you specify for storage pools on your cluster. The protection level is the actual protection that OneFS achieves for data, based on the protection policy and the actual number of writable nodes.

For example, if you have a three-node cluster, and you specify a protection policy of [+2d:1n], OneFS is able to tolerate the failure of two drives or one node without data loss. However, on that same three-node cluster, if you specify a protection policy of [+4d:2n], OneFS cannot achieve a protection level that would allow for four drive failures or two node failures. This is because N+M must be less than or equal to the number of nodes in the cluster.

By default, OneFS calculates and sets a recommended protection policy based on your cluster configuration. The recommended protection policy achieves the optimal balance between data integrity and storage efficiency.

You can set a protection policy that is higher than the cluster can support. In a four-node cluster, for example, you can set the protection policy at [5x]. However, OneFS would protect the data at 4x until you add a fifth node to the cluster, after which OneFS would automatically re-protect the data at 5x.