Re: Documentation for HHWM and LHWM?
Date: Tue, 6 May 2008 17:29:05 +0200
It has to do with how oracle process new allocated blocks. In case when hwm is moved forward, oracle does not immediately format all of the 'new' blocks allocated...oracle make it on their first use. This means, when full reading a segment, oracle somehow need to know if this blocks are formatted and eligible to be read or not...means nothing interesting in them. For that purpose there is low hwm and hwm (or hhwm). For all the blocks which are between low hwm and hwm, when full scanning a segment, oracle have to check assm bitmap block to see which blocks can be read and which can be skipped.
On Tue, May 6, 2008 at 4:43 PM, Charles Schultz <sacrophyte_at_gmail.com> wrote:
> Broswing through various blogs, whitepapers and other presentations, I have
> seen folks like Tom Kyte and Lutz Hartman reference the HHWM and LHWM of
> ASSM tables, however I have been unable to find where they are officially
> documented. Perhaps this is a limitation of tahiti and metalink (metalink
> gave me one hit for bug 6416822). I have looked through the Concepts, New
> Features and DBA books in the documentation set, learning a lot in the
> process, and still have a lot to go back over. *grin* Additionally, I
> appreciate that dbms_space gives us access to detailed information about
> used blocks, but I find it a little odd that the HWM is not specifically
> included, much less HHWM and LHWM. Forgive me if I missed it, but did Oracle
> ever document these ASSM high water marks?
> Thanks to all of you who have worked hard to expose various internal
> tidbits and publish helpful hints.
> Charles Schultz