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I have never heard of this but some crazy DBA Tool is telling me that maybe my DB_FILES are too high. Here is some basic info. Any help is appreciated.
DBWR Ave. Scan Depth = 1028.9114
db_files integer 100
thanks in advance,
Magnus
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After a bit of research,
DBWR Avg Scan Depth = (DBWR summed scan depth) / (DBWR lru scans)
Those values come from v$sysstat.
A quote I found from basically the same question was
<i>The DBWn scan depth is both a function of the scan depth boundaries and the write batch size. Further, if I recall correctly, the scan depth boundaries are primarily based on the write batch size too, and the db_block_buffers limit only applies if the write batch is very large. Of course, the default write batch size is a function of db_files and thus the comment (I assume).</i>
From [url]http://www.ixora.com.au/q+a/dbwn.htm[/url]
So basically because db_files determines write batch size, the scan depth boundries are large, causing a large DBWR summed scan depth. Divide that by a smaller DBWR lru scans and it gives the 1028 which I suppose is too high a ratio.
However I would wait for someone more informed to reply before changing anything :)
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That is pretty cool pwoneill. Btw, I was reading the oracle 24x7 chapter 7 and I noticed it to talk a lot on the
_DB_WRITER_SCAN_DEPTH : the number of buffers that DBWR will scan in the LRU list when looking for buffers. This is influenced by _DB_BLOCK_WRITE_BATCH
do this query and will show you all the hidden parameters and their values.
select ksppinm, ksppdesc, ksppstvl
from x$ksppi i,
x$ksppcv v
where v.indx = i.indx;
There are around 486 parameters and be v.v. carefull when playing with the uderscore parameters (not advisible!).
Sam
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