SMSE07_管理表空间和数据结构文件以及物理备份的基本概念.ppt

db file parallel write Reference Note This is a reference note for the wait event db file parallel write which includes the following subsections: Brief definition Individual wait details (eg: For waits seen in View:V$SESSION_WAIT) Systemwide wait details (eg: For waits seen in View:V$SYSTEM_EVENT) Reducing waits / wait times See Note:61998.1 for an introduction to Wait Events.Definition: Versions:7.0 - 9.2 Documentation:None This wait shows up in database writer. DBW waits on db file parallel write when waiting for a parallel write to files and blocks to complete. The wait lasts until all submitted IOs are complete. Individual Waits: ? Parameters: P1 = files P2 = blocks P3 = requests / timeout files Number of files This indicates the number of files that Oracle is writing to. blocks Number of blocks This indicates the total number of blocks to be written. requests / timeout Num requests / timeout From Oracle9.2 onwards P3 shows the timeout value in centiseconds to wait for the IO completion. Prior to Oracle9.2 this indicates the total number of I/O requests, which will be the same as P2 (blocks) ? Wait Time: The wait lasts until ALL of the submitted IO requests are complete. There is no Oracle timeout on this wait prior to Oracle9.2 . From 9.2 onwards the timeout is indicated by P3 ? Finding Blockers: This wait should only occur in database writer processes. The blocker is the Operating System IO subsystem. Systemwide Waits: If this is a significant portion of the total wait time it is not necessarily having a large impact on on user sessions. If user sessions show a large wait time on write complete waits and / or free buffer waits then this is impacting user sessions. A less obvious impact can be on the IO subsystem in that the writes may be impacting read times of sessions reading from the same disks. One can look at: View:V$FILESTAT to see where writes are occuring (although the write TIMES in this view are often wrong). Operating system IO s