RE: Why I get different 'db file scattered read' values?

From: Bobak, Mark <Mark.Bobak_at_il.proquest.com>
Date: Mon, 19 Jul 2004 13:57:23 -0400
Message-ID: <4C9B6FDA0B06FE4DAF5918BBF0AD82CF09660BF2@bosmail00.bos.il.pqe>

Lots of possible reasons.

Reads can be limited by (in no particular order):

1.)  Max I/O allowable by OS.
2.)  Max I/O allowable by filesystem, LVM, etc.
3.)  Max I/O allowable by Oracle on that platform.
4.)  Extent size.  (db file scattered read will not span extents.)
5.)  Block already in cache.  If Oracle is doing a read starting w/

block 1, for 8 blocks, but block 5 is in the cache, Oracle will only read 1-4, then do another read for 6-8.

In your case, I'd guess #5, but it's hard to be certain without more info....

-Mark
Mark J. Bobak
Oracle DBA
ProQuest Company
Ann Arbor, MI
"Post Hoc Ergo Propter Hoc"

-----Original Message-----
From: oracle-l-bounce_at_freelists.org
[mailto:oracle-l-bounce_at_freelists.org] On Behalf Of Juan Carlos Reyes Pacheco
Sent: Monday, July 19, 2004 1:34 PM
To: oracle-l_at_freelists.org
Subject: Why I get different 'db file scattered read' values?

Hi list, I runt the following script to get the DB_FILE_MULTIBLOCK_READ_COUNT value
=20

SQL> ALTER SESSION SET DB_FILE_MULTIBLOCK_READ_COUNT =3D100;
SQL> SET AUTOTRACE TRACEONLY
SQL> ALTER SESSION SET TRACEFILE_IDENTIFIER=3D'TEST_BLOCK_SIZE';
SQL> ALTER SESSION SET EVENTS '10046 TRACE NAME CONTEXT FOREVER, LEVEL
SQL> SELECT /*+ FULL(A) */ * FROM ADM.TRANSAC_ME A;

371196 filas seleccionadas.
Execution Plan

---

0 SELECT STATEMENT Optimizer=3DCHOOSE (Cost=3D206 Card=3D308952 Byte s=3D46342800)
=20

1 0 TABLE ACCESS (FULL) OF 'TRANSAC_ME' (Cost=3D206 Card=3D308952 Bytes=3D46342800)
Statistics

---

1118 recursive calls
0 db block gets
33023 consistent gets
8490 physical reads
60 redo size
31569813 bytes sent via SQL*Net to client 272709 bytes received via SQL*Net from client 24748 SQL*Net roundtrips to/from client
6 sorts (memory)
0 sorts (disk)
=20

But I get the different values in p3
WAIT #1: nam=3D'db file scattered read' ela=3D 32568 p1=3D3 p2=3D51689 = p3=3D8
WAIT #1: nam=3D'db file scattered read' ela=3D 15915 p1=3D3 p2=3D51778 = p3=3D7
WAIT #1: nam=3D'db file scattered read' ela=3D 16483 p1=3D3 p2=3D51785 = p3=3D8
WAIT #1: nam=3D'db file scattered read' ela=3D 29985 p1=3D3 p2=3D51937 = p3=3D8
WAIT #1: nam=3D'db file scattered read' ela=3D 150 p1=3D3 p2=3D52050 = p3=3D7
=20

Any idea?, my ntfs disk is compressed.
=20

Juan Carlos Reyes Pacheco
OCP

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Received on Mon Jul 19 2004 - 12:54:04 CDT