How Redo Send Works

ARCn Archival Processing

Archiving happens when a log switch occurs on the primary database:

* On the primary database, after one archival process (ARC0) has successfully archived the local online redo log to the local destination (LOG_ARCHIVE_DEST_1), another archival process (ARC1) process transmits redo from the local archived redo log files (instead of the online redo log files) to the remote standby destination (LOG_ARCHIVE_DEST_2).

* On the remote destination, the remote file server process (RFS) will, in turn, write the redo data to an archived redo log file from a standby redo log file. Log apply services use Redo Apply (managed recovery process - MRP ) or SQL Apply ( logical standby process - LSP uses parallel execution (Pnnn) processes) to apply the redo to the standby database.

Because the online redo log files are archived locally first, the LGWR process reuses the online redo log files much earlier than would be possible if the ARCn processes archived to the standby database concurrently with the local destination.

Using the Log Writer Process (LGWR) to Archive Redo Data

Using the LGWR process differs from ARCn processing because instead of waiting for the online redo log to switch at the primary database and then writing the entire archived redo log at the remote destination all at once, the LGWR process selects a standby redo log file at the standby site that reflects the log sequence number (and size) of the current online redo log file of the primary database. Then, as redo is generated at the primary database, it is also transmitted to the remote destination. The transmission to the remote destination will either be synchronous or asynchronous. Synchronous LGWR processing is required for the maximum protection and maximum availability modes of data protection in Data Guard configurations.

LGWR SYNC Archival Processing

LGWR process to synchronously transmit redo data to the standby system at the same time it is writing redo data to the online redo log file on the primary database:

* On the primary database, the LGWR process submits the redo data to one or more network server (LGWR Network Server process LNSn) processes, which then initiate the network I/O in parallel to multiple remote destinations. Transactions are not committed on the primary database until the redo data necessary to recover the transaction is received by all LGWR SYNC destinations.

* On the standby system, the remote file server (RFS) receives redo data over the network from the LGWR process and writes the redo data to the standby redo log files.

A log switch on the primary database triggers a log switch on the standby database, causing ARCn processes on the standby database to archive the standby redo log files to archived redo log files on the standby database. Then, Redo Apply (MRP process) or SQL Apply (LSP process) applies the redo data to the standby database. If real-time apply is enabled, Data Guard recovers redo data directly from the current standby redo log file as it is being filled up by the RFS process.


LGWR ASYNC Archival Processing

When the LGWR and ASYNC attributes are specified, the log writer process writes to the local online redo log file, while the network server (LNSn) processes (one for each destination) asynchronously transmit the redo to remote destinations. The LGWR process continues processing the next request without waiting for the LNS network I/O to complete.

If redo transport services transmit redo data to multiple remote destinations, the LNSn processes (one for each destination) initiate the network I/O to all of the destinations in parallel.

When an online redo log file fills up, a log switch occurs and an archiver process archives the log file locally, as usual.