Difference between LGWR SYNC and ASYNC in Oracle DataGuard

Oracle Data Guard redo log transport offers synchronous log transport mode (LogXptMode = 'SYNC') or asynchronous log transport mode (LogXptMode = 'ASYNC').  The difference is all about when the COMMIT happens . 

LogXptMode = ('SYNC'):  As the name implies, SYNC mode synchronizes the primary with the standby database and all DML on the primary server will NOT be committed until the logs have been successfully transported to the standby servers.  The synchronous log transport mode is required for the Maximum Protection and Maximum Availability data protection modes.

LogXptMode = ('ASYNC'): Conversely, asynchronous mode (ASYNC) allows updates (DML) to be committed on the primary server before the log file arrives on the standby servers.  The asynchronous log transport mode is required for the Maximum Performance data protection mode.

There is a very good post witten by "Shawn Kelley" related to Sync and Async in dataguard .

LGWR is an attribute of the LOG_ARCHIVE_DEST_n parameter which is used to specify the network transmission mode. Specifying the SYNC attribute (which is the default), tells the LGWR process to synchronously archive to the local online redo log files at the same time it transmits redo data to archival destinations. Specifically, the SYNC atrribute performs all network I/O synchornously in conjunction with each write operation to the online redo log file. Transactions are not committed on the primary database until the redo data necessary to recover the transactions is received by the destination.

The ASYNC attribute perfoms all network I/O asynchronously and control is returned to the executing application or user immediately. When this attribute is specified, the LGWR process archives to the local online redo log file and submits the network I/O request to the network server (LNSn process for that destination, and the LGWR process continues processing the next request without waiting for the network I/O to complete.

What happens if the network between the Primary and Standby [database] is lost with LGWR SYNC and ASYNC ? or What happens if the standby database is shutdown with LGWR SYNC and ASYNC?

This is dependent upon the database mode we have set. If we have set Maximum Protection, we have chosen a configuration that guarantees that no data loss will occur. We have set this up by specifying the LWGR, SYNC, and AFFIRM attributes of the LOG_ARCHIVE_DEST_n parameter for at least one standby database. This mode provides the highest level of data protection possible and to achieve this the redo data needed to recover each transaction must be written to both the local online redo log and the standby redo log on at least one standby database before the transaction commits. To ensure data loss cannot occur, the primary database shuts down if a fault (such as the network going down) prevents it from writing its redo stream to at least one remote standby redo log.

If  we have set the Maximum Availability mode, we have chosen a configuration that provides the highest level of data protection that is possible without compromising the availablity of the primary database. Like the maximum database does not shut down if a fault prevents it from writing its redo stream to a remote standby redo log. Instead, the primary database operates in maximum performance mode until the fault is corrected and all gaps in redo log files are resolved. When all gaps are resolved, the primary database automatically resumes operating in maximum availabitly mode. This guarantees that no data loss will occur if the primary database fails, but only if a second fault does not complete set of redo data being sent from the primary database to at least one standby database.

If we have set the Maximum Performance mode (the default), we have chosen a mode that provides the highest level of data protection that is possible without affecting the performance of the primary database. This is accomplished by allowing a transaction to commit as soon as the redo data needed to recover the transaction is written to the local online redo log. The primary database's redo data stream is also written to at least one standby database, bu that the redo stream is written asynchronously with respect to the commitment of the transactions that create the redo data.

The maximum performance mode enables us to either set the LGWR and AYSNC attributes, or set the ARCH attribute on the LOG_ARCHIVE_DEST_n parameter for the standby database destination. If the primary database fails, we can reduce the amount of data that is not received on the standby destination by setting the LGWR and ASYNC attributes.

If LGWR SYNC or ASYNC is deployed, what process(es) bring(s) the standby database back into sync with the primary [database] if the network is lost and is then restored? How does it do it?

Again, this is dependent upon the mode we have chosen for our database. The LGWR process (and possibly the LNSn process if we have multiple standby databases) is responsible for closing the gap.

My biggest question is, when the network to the standby is lost with SYNC or ASYNC, where is the information queued and how is it retransmitted once the network has been re-established?

This implies that our database has been set to either maximum availability or maximum performance mode. We cannot use the ASYNC attribute with maximum protection mode. The information is queued in the local online redo log and the LGWR (and the LNSn) process will transmit the data to the standby database's online redo log file to close the gap once the network connectivity has been re-established

Gap recovery is handled through the polling mechanism. For physical and logical standby databases, Oracle Change Data Capture, and Oracle Streams, Data Guard performs gap detection and resolution by automatically retrieving missing archived redo log files from the primary database. No extra configuration settings are required to poll the standby database(s) to detect any gaps or to resolve the gaps.

The important consideration here is that automatic gap recovery is contigent upon the availablity of the primary database. If the primary database is not available and we have a configuration with mulitple physical standby databases, we can set up additional initialization parameters so that the Redo Apply can resolve archive gaps from another standby database.

It is possible to manually determine if a gap exists and to resolve those archive gaps. To manually determine if a gap exists, query the V$ARCHIVE_GAP view on our physical standby database. If a gap is found, we will then need to locate the archived log files on our primary database, copy them to our standby database, and register them.

Enjoy       :)