  Large disk arrays are organized into storage nodes -- SNs or bricks with
their own cashed RAID controller for multiple disks. Erasure coding at SN level
is attained via parity or Reed-Solomon codes. Hierarchical RAID -- HRAID --
provides an additional level of coding across SNs, e.g., check strips P, Q at
intra-SN level and R at the inter-SN level. Failed disks and SNs are not
replaced and rebuild is accomplished by restriping, e.g., overwriting P and Q
for disk failures and R for an SN failure. For a given total redundancy level
we use an approximate reliability analysis method and Monte-Carlo simulation to
explore the better apportionment of check blocks for intra- vs inter-SN
redundancy. Our study indicates that a higher MTTDL -- Mean-Time-to-Data-Loss
-- is attained by associating higher reliability at intra-SN level rather than
inter-SN level, which is contrary to that of an IBM study.
