Data still in the buffer might not yet reside on the final recording medium. This is called data that is not synchronized. Data synchronization is the process by which the system ensures that data previously given to the system via WRITE, PUT, and PUTX macros is written to the storage medium.

When SYNCDEV completes successfully (return code 0), a value is returned that shows the number of data blocks remaining in the control unit buffer. For PDSEs and compressed format data sets, the value returned is always zero. For PDSEs and compressed format data sets, requests for synchronization information or for partial synchronization cause complete synchronization. Specify Guaranteed Synchronous Write through storage class to ensure that data is synchronized to DASD at the completion of each CHECK macro. However, this degrades performance. This produces the same result as issuing the SYNCDEV macro after each CHECK macro. See z/OS DFSMSdfp Storage Administration for information about how the storage administrator specifies guaranteed synchronous write.