Nodes in a recovery group

The number of nodes in a recovery group can also impact erasure code selection. If you consider a 4-node recovery group with 4+2P protection, each node contains 1 piece of data. In addition, for each stripe, 2 nodes contain 1 piece of parity data. A failure of a node that contains both parity and data results in a double failure for that stripe of data, which causes that stripe to be critical and results in performance degradation during the critical rebuild phase. However, in a 6-node recovery group, with the same 4+2P protection, a single node failure only results in 1 failure to the RAID array. Considering disk failures and spare disk space for rebuild, a 7 or more-node recovery group is recommended for 4+2P erasure code, it can keep fault tolerance after rebuilding the data to spare space in case of disk failure. For more information, see RAID rebuild and Spare space.