When a user commits a transaction, the transaction is assigned a system change
number (SCN), which Oracle records along with the transaction’s redo entries in the
redo log. SCNs are recorded in the redo log so that recovery operations can be
synchronized in Real Application Clusters and distributed databases.
In times of high activity, LGWR can write to the redo log file using group commits. For
example, assume that a user commits a transaction. LGWR must write the
transaction’s redo entries to disk, and as this happens, other users issue COMMIT
statements. However, LGWR cannot write to the redo log file to commit these
transactions until it has completed its previous write operation. After the first
transaction’s entries are written to the redo log file, the entire list of redo entries of
waiting transactions (not yet committed) can be written to disk in one operation,
requiring less I/O than do transaction entries handled individually. Therefore, Oracle
minimizes disk I/O and maximizes performance of LGWR. If requests to commit
continue at a high rate, then every write (by LGWR) from the redo log buffer can
contain multiple commit records.
当用户提交一个事务时 , 将被记录一个SCN , 同样也记录在重做日志文件中 .
当系统处于高数位活动量时 , LGWR可以使用名为组提交的机制来减少磁盘I/O .
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