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Definition of wait event [message #343639] Wed, 27 August 2008 13:13 Go to next message
lunaut
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Registered: July 2008
Junior Member
The definition of WAITS is not quite clear to me. As I read from an article : At any given Moment, every Oracle process is either serving a request or waiting for something specific to happen. If a process is searching the buffer cache for certain data block, it is busy. The ARCO process is said waiting when it is waiting for LGWR to signal that an online redo log needs archive.

Then if a process is searching/loading data blocks from disk, is it also BUSY, or waiting? I guess it is waiting, as there is a waiting class 'User IO'. Why serching buffer cache is BUSY while searching/loading disk is WAIT? Just because disk IO take longer?

I have a session which has the wait events ‘db file sequential read’ and the wait class is User I/O. Does the session wait for the data blocks it requires to load to the memory from disk, or its request is waiting in queue for other process’s disk IO to finish.

If it is waiting for disk IO of other processes, shall we say that is BLOCKING? If so we need to tune sql statement of the other processes. But how to find the sessions/queries that are waited for?

Thanks
Re: Definition of wait event [message #343642 is a reply to message #343639] Wed, 27 August 2008 13:18 Go to previous messageGo to next message
BlackSwan
Messages: 25046
Registered: January 2009
Location: SoCal
Senior Member
http://www.orafaq.com/forum/t/88153/0/
Please read & follow posting guidelines as stated in URL above

> But how to find the sessions/queries that are waited for?


AWR report(s)
Re: Definition of wait event [message #343648 is a reply to message #343639] Wed, 27 August 2008 13:46 Go to previous messageGo to next message
Michel Cadot
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Location: Nanterre, France, http://...
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Same answer than in your previous topic: What do Oracle waits mean

Regards
Michel
Re: Definition of wait event [message #344071 is a reply to message #343639] Thu, 28 August 2008 13:06 Go to previous messageGo to next message
lunaut
Messages: 6
Registered: July 2008
Junior Member
I read the guild and believe my post followed it.

How can I get a simple answer on 'A wait event means a session is waiting for its own job, like disk IO, or waiting for other's session?'?

I did search the docs, and web, some docs imply that the wait is on it self's action. But would that means BUSY?
Re: Definition of wait event [message #344074 is a reply to message #344071] Thu, 28 August 2008 13:09 Go to previous messageGo to next message
BlackSwan
Messages: 25046
Registered: January 2009
Location: SoCal
Senior Member
lunaut wrote on Thu, 28 August 2008 11:06
I read the guild and believe my post followed it.



Post exact Database (4 decimal places, please) and OS version among others
Re: Definition of wait event [message #344077 is a reply to message #344071] Thu, 28 August 2008 13:16 Go to previous message
Michel Cadot
Messages: 64130
Registered: March 2007
Location: Nanterre, France, http://...
Senior Member
Account Moderator
Each process is either:
- non-existent
- busy
- waiting

If it exists and is not executing cpu instructions then it is waiting, if it is waiting for something then it is not busy.
Do this answer your question?

Regards
Michel
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