Oracle Study--oracle Wait event (3)

Oracle Study--oracle Wait event (3)

Db file Parallel read
This is a very misleading wait event, and in fact this wait event is not related to parallel operations such as parallel queries, parallel DML. This event occurs when the database is recovering, and when some data blocks need to be restored, Oracle reads them from the data file into memory in parallel to restore the operation.
This wait event consists of three parameters:
files: The number of documents that the operation needs to read.
Blocks: The number of data blocks that the operation needs to read.
requests: The number of I/Os that the operation needs to perform.

case study:

11:06:16 [email protected] prod>conn scott/tigerconnected.11:06:19 [email  Protected] prod>insert into emp1 select * from emp1;286720 rows  created. elapsed: 00:00:02.0411:07:22 [email protected] prod>/573440 rows created.11:0 8:07 [email protected] prod>r  1  select event,total_waits,average_ Wait from v$system_event  2* where upper (event)  like  ' db file% ' event                                                               total_waits average_ WAIT---------------------------------------------------------------- ----------- ------------db file sequential read                                                    4449           .04db file scattered read                                                     1650          .02db file  single write                                                        36           .39db file async I/O submit                                                    133          8.69db file parallel read                                                        37          .19elapsed:  00:00:00.0211:08:20 [email protected] prod>r  1  select event,total _waits,average_wait from v$system_event  2* where upper (event)  like  ' Db file% ' event                                                               total_waits average_ WAIT---------------------------------------------------------------- ----------- ------------DB  file sequential read                                                   7561           .03db file scattered read                                                     1785          .02db  file single write                                                         36          .39db file async i/o submit                                                    160         10.29db file parallel read                                                        39           .19elapsed: 00:00:00.02

Db file Parallel write
This is a background wait event, it is also not related to the user's parallel operation, it is generated by the background process DBWR, when the background process DBWR to the disk to write dirty data, this wait occurs.
DBWR will write the dirty data in parallel to the corresponding data file on disk, and DBWR will appear this wait event until the batch job is completed. If this is just a wait event, there is not much impact on the user's operation, when there is a free buffer waits waiting event, it indicates that there is not enough space available in memory at this time, it will affect the user's operation, such as affecting the user to read the dirty block into memory.
when a DB file parallel write wait event occurs, this wait can be mitigated by enabling the operating system's asynchronous I/O. When using asynchronous I/O, DBWR no longer needs to wait until all blocks of data are written to disk, and it only needs to wait until the data is written to a percentage before it can proceed with subsequent operations.
This wait event has two parameters:
Requests: The number of I/Os that the operation needs to perform.
Timeouts: The wait time-out period.

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