RE: Extents size.

From: MacGregor, Ian A. <ian_at_SLAC.Stanford.EDU>
Date: Wed, 03 Oct 2001 14:40:00 -0700
Message-ID: <F001.003A1363.20011003145018@fatcity.com>

This is very noticeable when you are debugging a problem and are seeing "scattered read" events. The query against dba_EXTENTS based on file_id, block_id and owner can take an inordinate amount of time.

Ian MacGregor
Stanford Linear Accelerator Center
ian_at_slac.stanford.edu

-----Original Message-----
Sent: Wednesday, October 03, 2001 8:25 AM To: Multiple recipients of list ORACLE-L

Christopher,

Is the guideline 505 extents for the tablespace or each table in the tablespace?

Tom

Tom Terrian
Oracle DBA
WPAFB - DAASC
tterrian_at_daas.dla.mil
937-656-3844

-----Original Message-----
Sent: Wednesday, October 03, 2001 10:05 AM To: Multiple recipients of list ORACLE-L

LMT's have their own problems as well though.

Using LMT's, the bitmap on the tablespace is only used to manage free space, used space is in the segment header of the segment which represents the extents. Therefore, to do a query of DBA_EXTENTS you can hit all segment headers (of all tables and indexes in the database) as well as all the additional extent maps. This can cause a very large amount of physical reads in comparison to the dictionary-managed extents.

Although LMT's are great, they are not perfect and do have their share of concerns. And like Connor pointed out, for a 8K block size, it is highly suggested to keep the extents to 505 which would only create a single block for the segment header, anything further than that will create additional blocks for the segment. Remember, DBA_EXTENTS has to hit all the segment headers (be it one or two) as well as the extent maps.

The formula he used is (blocksize / 16)-7, which determine how many extents, can fit in the segment header of a LMT tablespace.

"Do not criticize someone until you walked a mile in their shoes, that way when you criticize them, you are a mile a way and have their shoes."

Christopher R. Spence
Oracle DBA
Phone: (978) 322-5744
Fax: (707) 885-2275

Fuelspot
73 Princeton Street
North, Chelmsford 01863

-----Original Message-----
Sent: Tuesday, October 02, 2001 5:55 PM
To: Multiple recipients of list ORACLE-L

True - but we're all using LMT's now anyway aren't we ?

:-)

Rachel Carmichael <wisernet100_at_yahoo.com> wrote:
> well, only when you are deleting massive amounts of
> data or truncating
> with drop storage.. then there is an impact because
> of the hits on the
> dictionary tables.
>
> but basically yes... I've been told by various
> Oracle employees that up
> to 4096 extents cause no problem whatsoever.
>
>
> --- Christopher Spence <cspence_at_FuelSpot.com> wrote:
> > That is completely a myth. There is no notable
> performance different
> > with a
> > table with 10,000 extents and one with 1.
> >
> > The only problem is when it comes to the bitmaps
> when dealing with
> > LMT and
> > cluster when dealing with dictionary managed.
> When you query the
> > extent
> > views, or do space management type processes.
> >
> > "Do not criticize someone until you walked a mile
> in their shoes,
> > that way
> > when you criticize them, you are a mile a way and
> have their shoes."
> >
> > Christopher R. Spence
> > Oracle DBA
> > Phone: (978) 322-5744
> > Fax: (707) 885-2275
> >
> > Fuelspot
> > 73 Princeton Street
> > North, Chelmsford 01863
> >
> >
> >
> > -----Original Message-----
> > Sent: Monday, October 01, 2001 7:15 PM
> > To: Multiple recipients of list ORACLE-L
> >
> > May be it is good practice to keep number of
> extents to be less than
> > 50, no
> > matter what the size of extent.
> >
> >
> >
> > -----Original Message-----
> > Sent: Monday, October 01, 2001 3:35 PM
> > To: Multiple recipients of list ORACLE-L
> >
> >
> > Hello,
> >
> > I'll do an reorganization of a database (about 140
> gigs). Some people
> > say
> > that it'd be good to use 128K, 4M and 128M
> extents. I saw somewhere
> > it'd be
> > 160K, 4M and 160M. Which size do you advice me ? I
> have also many
> > small
> > indexes (less than 16K).
> >
> > Regards,
> >
> > Thanh-truc Nguyen
> >
> > --
> > Please see the official ORACLE-L FAQ:
> http://www.orafaq.com
> > --
> > Author: Thanh-truc Nguyen
> > INET: truc_at_nguyen.as
> >
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Author: MacGregor, Ian A.
  INET: ian_at_SLAC.Stanford.EDU

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Received on Wed Oct 03 2001 - 16:40:00 CDT