Oracle Forms

In general each session writes its own redo into private buffers (a matched pair - one for the forward changes, one for the undo block changes - it's the latter that's named the IMU pool) and copies the private redo into the public log buffer on commit. There are various events that will cause the private redo to be flushed to the public thread before the commit. One of these is the special case that you've mentioned where DBWR instigates a flush.

If DBWR has to copy a datablock from the db cache to disc it has to copy the latest version of the block, but any redo that describes changes to that block has to be written first. So if DBWR finds that a block it wants to write is subject to private redo it flushes the redo to the log buffer and applies it to the data buffer before calling lgwr to write the log buffer to disc - once the log writer has written dbwr can write that data block to disc.


Private strand flush incomplete is the event you get from a foreground session when something else (such as DBWR) is doing the flush for it and it's waiting to generate more redo. It can't put it into the private buffer because that buffer is now locked against change because of the flush; it can't put it into the public buffer otherwise the redo might get there before the sessions earlier redo, so it has to wait temporarily for the private buffer to become unlocked because that's the only way it can tell that the flush is complete and it's now safe to carry on - and when it carries on it has to switch to the public buffer.

I've just finished rewriting half a dozen pages of my book explaining this, so a short answer that hits a few of the highlights is all you can expect in a forum note.

Regards
Jonathan Lewis
http://jonathanlewis.wordpress.com
http://www.jlcomp.demon.co.uk

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