InnoDB crash recovery consists of several
steps. The first step, redo log application, is performed during
the initialization, before accepting any connections. If all
changes were flushed from the buffer pool to the tablespaces
files) at the time of the shutdown or crash, the redo log
application can be skipped. If the redo log files are missing at
InnoDB skips the redo log
The remaining steps after redo log application do not depend on the redo log (other than for logging the writes) and are performed in parallel with normal processing. These include:
Rolling back incomplete transactions: Any transactions that were active at the time of crash or fast shutdown.
Insert buffer merge: Applying changes from the insert buffer tree (from the shared tablespace) to leaf pages of secondary indexes as the index pages are read to the buffer pool.
Purge: Deleting delete-marked records that are no longer visible for any active transaction.
Of these, only rollback of incomplete transactions is special to crash recovery. The insert buffer merge and the purge are performed during normal processing.