The redo queue is the queue of log records on the mirror, log records that have already been hardened on the principal and mirror and only need to be redone (against the data file) on the mirror. The redo queue must be completely redone before the mirror comes online.
You won't have unsent log in synchronous mirroring unless the mirror is not in the synchronised state (due to latency or line failure)
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability