Resolution:
When syncLedger is set to true, when RV opens the ledger file, the O_SYNC or O_DSYNC mode is specified. This makes all the write() to ledger file become synchronous calls, i.e., the write() will not return until the data is actually persistent on disk. This is equivalent to the default $sys.failsafe store in EMS.

When syncLedger is set to false, no other file mode is specified other than O_CREAT | O_RDWR, therefore all write() calls to the ledger file are asynchronous. The write() call will return immediately without having to wait for the data to be persisted to disk. This is similar to the $sys.nonfailsafe store in EMS.