Explain about high water mark and low watermark

explain about high water mark and low watermark?

biju2012 wrote:
Here are a few links..
http://www.orafaq.com/tuningguide/
Look at Analysing problem SQL > Data Storage Problems > High Water Mark
http://asktom.oracle.com/pls/asktom/f?p=100:11:0::::P11_QUESTION_ID:575223085419
I checked both those links, and neither says anything about a "low water mark". One link has a copyright notice of 2003, the other starts with a question about 8.1.6.
Given the nature of the question I think we should be allowed to guess that the OP is probably thinking about the "low high water mark" and "high high water mark" that applies to segments using ASSM (automatic segment space management).
Under ASSM, Oracle doesn't format blocks for use in absolute order, it will pick batches of blocks (typically 16 - unless that's changed fairly recently) in the most recently allocated extent. This means that you can end up with some UNFORMATTED blocks near the start of the extent with some FORMATTED blocks later in the extent.
The "low high watermark" is the last block below which there are no UNFORMATTED blocks; the "high high watermark" is the block above which there are no FORMATTED blocks. Looking at it another way - between the low and high high watermarks you may find both formatted and unformatted blocks, but this condition doesn't exist anywhere else in the segment.
Regards
Jonathan Lewis

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