Re: buffer busy waits and v$filestat

Hello Raj.

BBW with a p3=0 are a consecuence of the I/O subsystem not being able to provide enough throughput to the database, as Mladen has said.
But there are also many others causes for BBW. Check p3.

Also if the session A is waiting for a buffer in the buffer cache (that's a BBW), the buffer is already in the BC so this session has no need (won't) to do a physical I/O for that block.

Another thing I think (I'm sorry to disagree with Mladen on this) is that when DBWR hasn't finished writing a buffer to the disk, and a session wants that buffer in exclusive mode, there's a wait and that wait is computed as a write complete wait and not as BBW.

please correct me if I'm wrong.

thanks
HTH
Greetings
Diego Cutrone

>Folks,
>
>Say a session issues a read request, and finds
>another session already
>reading the block into the buffer cache. If this
>session waits N ms on a
>"buffer busy waits" event, does this N ms of wait get
>added to the read
>times in v$filestat? Or is the readtim in v$filestat
>purely physical I/O?
>
>Thanks
>Raj
>
>

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