Synchronous and Synchronized Operations

Synchronous means the IO operation does not return control to the caller.  Synchronized operation means the operation ensures data are up to date (on disk).

When synchronous write is synchronized, the write will not return until data is flushed to disk.  This is equivalent of O_SYNC flag effect when opening file.

When synchronous write is not synchronized, the write will return when the data is copied to the kernel buffer.  So the disk copy is different from the memory copy.  This is the default effect of Linux write operation.

When asynchronous write is synchronized, the control return to caller as soon as the request is queued.  When the write is later executed, the data is guaranteed to be written to disk.

When asynchronous write is not synchronized, the control is returned immediately after the request has been queued.  The data is guaranteed to store in kernel buffer only and so the disk copy could be different from the memory copy.

Read is always synchronized (i.e. the data return is always up to date).  Synchronous or asynchronous read call determine when control is returned.