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Home -> Community -> Mailing Lists -> Oracle-L -> FW: Antwort: RE: Antwort: RE: Buffer Busy Waits -- Sanity check
The table has about 550,000 rows, and will be growing, currently about 230MB in size.
-----Original Message-----
From: Ivo.Libal_at_knapp-systems.com [mailto:Ivo.Libal_at_knapp-systems.com]
Sent: Tuesday, November 27, 2001 7:25 AM
To: Multiple recipients of list ORACLE-L
Subject: Antwort: RE: Antwort: RE: Buffer Busy Waits -- Sanity check
please
But what if they are aging out, isnt that one of reasons for buffer busy and they are - if there is a scattered read....
From docus:
As buffer busy waits are due to contention for particular blocks then you
cannot take any action until you know which blocks are being competed for
and why. Eliminating the cause of the contention is the best option. Note
that "buffer busy waits" for data blocks are often due to several processes
repeatedly reading the same blocks (eg: if lots of people scan the same
index) - the first session processes the blocks that are in the buffer
cache quickly but then a block has to be read from disk - the other
sessions (scanning the same index) quickly 'catch up' and want the block
which is currently being read from disk - they wait for the buffer as
someone is already reading the block in.
If the table s not too big, than I would put it to the cache to avoid
reloads from harddisk.
Ivo
Design & Development, Warehousemanagementsystems Dept.
KNAPP Systemsintegration GmbH
Waltenbachstraße 9
A-8700 Leoben, Austria
Phone: ++43/3842/805-0
e-Mail: ivo.libal_at_knapp-systems.com
"Thorns, Malcolm (NESL-IT)"
<Malcolm.Thorns_at_northern-elect An: Multiple recipients of list ORACLE-L <ORACLE-L_at_fatcity.com>
ric.co.uk> Kopie:
Gesendet von: root_at_fatcity.com Thema: RE: Antwort: RE: Buffer Busy Waits -- Sanity check please
11/27/2001 11:10 AM
Bitte antworten an ORACLE-L
This will not resolve the 'buffer busy wait' scenario. The contention is for blocks that are already in the SGA.
Regards,
Malcolm.
-----Original Message-----
Sent: Tuesday, November 27, 2001 9:15 AM
To: Multiple recipients of list ORACLE-L
Hi
I would try to set the table to cache mode. It seams that the blocks are
aged out from buffer - which is default for FTS.
alter table BBN_SRVvignette cache;
to avoid reloading into cache.
Ivo
Design & Development, Warehousemanagementsystems Dept.
KNAPP Systemsintegration GmbH
Waltenbachstraße 9
A-8700 Leoben, Austria
Phone: ++43/3842/805-0
e-Mail: ivo.libal_at_knapp-systems.com
"Thorns, Malcolm (NESL-IT)"
<Malcolm.Thorns_at_northern-elect An: Multiple
recipients of list ORACLE-L <ORACLE-L_at_fatcity.com>
ric.co.uk> Kopie:
Gesendet von: root_at_fatcity.com Thema: RE:
Buffer
Busy Waits -- Sanity check please
11/27/2001 09:25 AM
Bitte antworten an ORACLE-L
Jeff,
The 3 sessions are doing the same (or similar) queries. In this case count(*) which is forcing a full table scan of the table in each session. The 3 sessions are thus trying to access the same blocks from the SGA, in the same order. Only 1 session can access a block in the SGA at a time - this is the session showing 'db file scattered read'. The other 2 sessions need to wait for the block (these waits show as 'buffer busy waits' - ie waiting for the block in the SGA). You will see the block id (and perhaps the file id) changing as the FTS's progress. Thus the sessions are 'chasing' each other through the blocks - holding each other up with SGA block contention - which shows up as 'buffer busy waits'. Hope that explains things.
Regards,
Malcolm
-----Original Message-----
Sent: Monday, November 26, 2001 11:21 PM
To: Multiple recipients of list ORACLE-L
We recently had a new website go live. Since then, I'm seeing constant
buffer busy waits
and after a period of time, I see sessions hung on the same block#. The
SQL query
is always a COUNT(*) (below). It's almost as though one session has a
lock
of some sort in the buffer cache and other sessions are blocked.
Although,
I've checked and
there's no DML ongoing, so I'm unsure as to why we would see this. Note
that v$session shows
78 and 393 to be INACTIVE, while 159 is ACTIVE. So it's like 159 can't
write to
the buffer cache because 78 and 393 have a lock there. Note that these
are
all defined
as persistent connections, via the Vignette front-end. I'm sure all the
clues are there
but my brain is too fuzzed to piece it together.
SID SQL_TEXT O/S
User
----- ----------------------------------------------------------------
SID EVENT P1TEXT P1 P2TEXT P2 P3TEXT
P3
----- -------------------------- ------------ -- ----------- ------ -----
Email: thomasje_at_tce.com
DBA Quickplace: http://gkmqp.tce.com/tis_dba <http://gkmqp.tce.com/tis_dba>
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-- Please see the official ORACLE-L FAQ: http://www.orafaq.com -- Author: Thorns, Malcolm (NESL-IT)Received on Tue Nov 27 2001 - 08:56:46 CST
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