
OneFS 145
of size N
0
+ M where N
0
= 2N + M. These double-length protection groups
will “wrap” once over the same set of nodes, as they are laid out. Clearly,
since each protection group contains exactly two disks’ worth of redundancy
(M = 2), this mechanism will allow for the double-drive failure. When a node
fails, it causes two erasures in the protection group, but again M = 2 will
provide double-disk redundancy.
Most important for small clusters, this method of striping is highly effi-
cient, with an on-disk efficiency of 1 − Mb(N + M). For example, on a cluster
of five nodes with double-failure protection, were one to use N = 3, M = 2,
and b = 1, one would ...