Log Writer Process (LGWR)

Log Writer Process (LGWR)

The log writer process (LGWR) manages the redo log buffer. LGWR writes one contiguous portion of the buffer to the online redo log. By separating the tasks of modifying database buffers, performing scattered writes of dirty buffers to disk, and performing fast sequential writes of redo to disk, the database improves performance.

In the following circumstances, LGWR writes all redo entries that have been copied into the buffer since the last time it wrote:

• A user commits a transaction (see "Committing Transactions").

• An online redo log switch occurs.

• Three seconds have passed since LGWR last wrote.

• The redo log buffer is one-third full or contains 1 MB of buffered data.

• DBWn must write modified buffers to disk.

  Before DBWn can write a dirty buffer, redo records associated with changes to the buffer must be written to disk (the write-ahead protocol). If DBWn finds that some redo records have not been written, it signals LGWR to write the records to disk and waits for LGWR to complete before writing the data buffers to disk.

LGWR and Commits

Oracle Database uses a fast commit mechanism to improve performance for committed transactions. When a user issues a COMMIT statement, the transaction is assigned a system change number (SCN). LGWR puts a commit record in the redo log buffer and writes it to disk immediately, along with the commit SCN and transaction's redo entries.

The redo log buffer is circular. When LGWR writes redo entries from the redo log buffer to an online redo log file, server processes can copy new entries over the entries in the redo log buffer that have been written to disk. LGWR normally writes fast enough to ensure that space is always available in the buffer for new entries, even when access to the online redo log is heavy.

The atomic write of the redo entry containing the transaction's commit record is the single event that determines the transaction has committed. Oracle Database returns a success code to the committing transaction although the data buffers have not yet been written to disk. The corresponding changes to data blocks are deferred until it is efficient for DBWn to write them to the data files.

Note:

LGWR can write redo log entries to disk before a transaction commits. The redo entries become permanent only if the transaction later commits.

When activity is high, LGWR can use group commits. For example, a user commits, causing LGWR to write the transaction's redo entries to disk. During this write other users commit. LGWR cannot write to disk to commit these transactions until its previous write completes. Upon completion, LGWR can write the list of redo entries of waiting transactions (not yet committed) in one operation. In this way, the database minimizes disk I/O and maximizes performance. If commits requests continue at a high rate, then every write by LGWR can contain multiple commit records.

LGWR and Inaccessible Files

LGWR writes synchronously to the active mirrored group of online redo log files. If a log file is inaccessible, then LGWR continues writing to other files in the group and writes an error to the LGWR trace file and the alert log. If all files in a group are damaged, or if the group is unavailable because it has not been archived, then LGWR cannot continue to function.

See Also:

• "How Oracle Database Writes to the Online Redo Log" and "Redo Log Buffer"

• Oracle Database Performance Tuning Guide for information about how to monitor and tune the performance of LGWR

Checkpoint Process (CKPT)

The checkpoint process (CKPT) updates the control file and data file headers with checkpoint information and signals DBWn to write blocks to disk. Checkpoint information includes the checkpoint position, SCN, location in online redo log to begin recovery, and so on. As shown in Figure 15-4, CKPT does not write data blocks to data files or redo blocks to online redo log files.

Figure 15-4 Checkpoint Process

Description of "Figure 15-4 Checkpoint Process"

See Also:

"Overview of Checkpoints"

from：http://docs.oracle.com/cd/E11882_01/server.112/e40540/process.htm#CNCPT1251