Mirroring, however, consumes more space than erasure codes. Mirroring data three times, for example, duplicates the data three times, which requires more space than erasure codes. As a result, mirroring suits transactions that require high performance.
You can also mix erasure codes with mirroring. During a write operation, OneFS divides data into redundant protection groups. For files protected by erasure codes, a protection group consists of data blocks and their erasure codes. For mirrored files, a protection group contains all the mirrors of a set of blocks. OneFS can switch the type of protection group as it writes a file to disk. By changing the protection group dynamically, OneFS can continue writing data despite a node failure that prevents the cluster from applying erasure codes. After the node is restored, OneFS automatically converts the mirrored protection groups to erasure codes.