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Home -> Community -> Mailing Lists -> Oracle-L -> Re: delayed logging block cleanout
Hi Jared,
>From the link you pointed me out (Steve Adams response):
"
Delayed block cleanout is performed by the next reader of the block, even if
it is just a query. The block is read into the cache as a current mode block
and cleaned out. If a query is being performed, a consistent read mode clone
of the block is made using another block buffer, and if necessary rolled
back to the snapshot SCN for the query. The consistent read mode buffer is
used to satisfy the query. However, any changes are made to the current mode
buffer.
Because block cleanout actually makes changes to the current mode buffer, it must generate redo. If delayed_logging_block_cleanouts is TRUE, which is the default in 7.3 and 8.0, the logging of this redo is delayed until another change is made to the current buffer. The cleanout itself does not mark the buffer as dirty or log the redo for its changes. The next change to the buffer marks the buffer as dirty and logs redo for the cleanout as well as its own changes.
If the parameter is FALSE, the cleanout logs its own redo and marks the buffer as dirty, even if the block was only being used for a query. Because the buffer is dirty, DBWn must write the cleaned out block back to disk. This increases DBWn's workload, but prevents the cleanout from having to be repeated by later queries.
"
Jared, maybe I'm getting it wrong, but I remember I having tested this once
in an Oracle 7 instance and I got the results Steve remarks here (same thing
I've said in my previous email).
In Oracle 7 (the default behavior) not only the logging is delayed but the
whole block clean out is delayed until another *transaction* takes place in
the same block.
Now, if a *query* is executed (instead of a transaction), this query would
have to clean out the blocks but (as Steve says) these cleaned blocks are
*not* written to disk, so another query would have to do the same clean out
until a transaction takes place in that block and cleans it out definitely.
This is *not* what I've seen in the tests against a 7 and 8.0 database I have performed today. If you check it out, the block got completely cleaned when I executed a query (the default behavior in Oracle 8i)
Thanks for answering
Diego.
> Diego,
> The d_l_l_c parameter does not delay the cleanouts, just the logging of
> them.
>
> See http://www.ixora.com.au/q+a/cr.htm
>
> Jared
>
-- Archives are at http://www.freelists.org/archives/oracle-l/ FAQ is at http://www.freelists.org/help/fom-serve/cache/1.html -----------------------------------------------------------------Received on Wed May 26 2004 - 16:29:19 CDT
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