Not completely true.
If the dbwr is going to write a buffer, it will set a bit that the buffer is
being written. In the good old days, this meant that the buffer could be
changed until the block was written ('write complete waits'). However in 8.1,
cloning of buffers was introduced. So now the dirty buffer is cloned (see
stats in v$sysstat), the new clone can be changed, the old clone is written.
(old and new are bad words, but you get my drift). So write complete waits
should be a whole lot less in 8.1 then before. They may still happen (not
sure why yet).
Anjo.
On Wednesday 30 July 2003 03:04, you wrote:
> That's On 2003.07.29 19:59, Diego Cutrone wrote:
> > 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.
>
> DBWR works in 2 phases:
> a) It scans for dirty buffers and pins them.
> b) It starts IO, usually using writev.
>
> If IO has been launched and not yet finished, then the wait is
> "write complete wait". If IO hasn't been started yet, we have "buffer
> busy". This "write complete" wait became essential with the advent of
> asynchronous I/O. I was just simplifying things, nothing else.
--
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