_wait_for_sync , dirty buffer flushing and direct reads in parallel query

Hi!

I've sometimes used setting _wait_for_sync=false during Apps upgrade projects, to upgrade performance. (As long as your database doesn't crash during the parameter is set to false, no problems should occur).

I just started wondering, what would be the case if a parallel query starts during someone is modifying data...

As I understand, when doing parallel query: 1) the dirty blocks which are supposed to be read by PQ in direct mode, are flushed to disk
2) PQ reads the blocks in direct mode

But when _wait_for_sync is set, the writes get acknowledged immediately (or acknowledgement is not waited for). Could this result in the unlikely situation, that PQ issues the flush command to dirty buffers and starts to read them, but actually reads the old images of the blocks, since it thinks the write has already occurred?

(actually, this doesn't touch only PQ, it's possible to have direct reads to
PGA in serial mode too...)

Tanel.

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Please see the official ORACLE-L FAQ: http://www.orafaq.net
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Author: Tanel Poder
  INET: tanel.poder.003_at_mail.ee

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