A system and method for fault-tolerant synchronization of replica updates
for fixed persistent consistency point image consumption. PCPIs are
consumed at a fixed level and each coalesced PCPI presents a valid
version of the source file system from some point in time. The qtrees
concurrently move through a coalescing cycle wherein some qtrees are
updated, some may update partially and some may not update at all.
Partially updated qtrees are rolled-back to the previous state, after
which all qtrees are transitioned to a stable state.