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Re: PCTFREE and PCTUSED

From: Richard Foote <richard.foote_at_bigpond.com>
Date: Fri, 07 Nov 2003 14:59:39 -0800
Message-ID: <F001.005D6028.20031107145939@fatcity.com>


Hi (again) Mladen,

I'm sure I mentioned this previously but ASSM only deals with FREELISTS, FREELIST GROUPS and PCTUSED (with possibly significant overheads).

You still need to set *PCTFREE*, which means you can still have over allocation of space if you set it too high, you can still have row migration if you set it too low, you still have row chaining, you can still have waits on ITL entries and other lovely things ...

ASSM is most certainly *not* some magic fix.

And it's only available since 9i.

Cheers

Richard

> On the other hand, you might have overallocated the space, which would
> leave plenty of blocks on the free list, thus minimizing the impact.
> These things are best seen on "almost full" tables with things like
> row chaining, row migration, waits on ITL entries and other lovely
> things. Looks like you've benn lucky so far. As I've told you before,
> having tablespaces created with "SEGMENT SPACE MANAGEMENT AUTO" option
> takes care of that.
> if your tablespace is created with a command like
>
> "CREATE TABLESPACE DATA01
> DATAFILE '/data01/SID1/data01.dbf' size 8192M reuse
> AUTOEXTEND ON NEXT 4096M MAXSIZE 32769M
> EXTENT MANAGEMENT LOCAL AUTOALLOCATE
> SEGMENT SPACE MANAGEMENT AUTO"
>
> then you can create tables without any additional parameters and
everything will be kosher.
> The tablespace above requires something called "large files support" from
the file system
> and cannot be used on FAT file systems or ISO9660-RR/Joliet file system.
Practically anyhing
> else (UFS,VxFS,ReiserFS,Ext(2|3),XFS,JFS,OCFS,NTFS (scheduled to go away
when the new virus
> propagataion engine enters production)).
> BTW, what OS and database version do you use?
>
>
> On 11/07/2003 01:24:25 PM, Maryann Atkinson wrote:
> >
> > >The answer is that
> > >free list handling is overhead, which means that the database is
working on
> > >its own structures and not working on the user data. It's easy to
conceive a
> > >busy transaction table to which records are frequently added and from
which
> > >they're frequently removed. Having only one parameter would
significantly
> > >increase the amount of time spent in moving blocks to and from the free
> > >list,
> > >and significantly increase the overhead. You can test it by setting up
a
> > >table
> > >with PCTFREE+PCTUSED=100. In other word, the answer to your question is
that
> > >two parameters are needed to reduce the overhead of the free list
maintenance.
> >
> > By the way, I just wanted to let you know I tried that
PCTFREE+PCTUSED=100
> > I used PCTFREE 40 and PCTUSED 60 on a specific table space, and created
> > a table there which I updated/deleted records quite a bit, but I didnt
see
> > much
> > of a difference. It might have been because there werent many people
> > on the system at the time.
> >
> > thx
> > maa
> >
> >
> > --
> > Please see the official ORACLE-L FAQ: http://www.orafaq.net
> > --
> > Author: Maryann Atkinson
> > INET: maryann_30_at_yahoo.com
> >
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> >
>
> Mladen Gogala
> Oracle DBA
>
>
>
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> --
> Please see the official ORACLE-L FAQ: http://www.orafaq.net
> --
> Author: Mladen Gogala
> INET: mladen_at_wangtrading.com
>
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-- 
Please see the official ORACLE-L FAQ: http://www.orafaq.net
-- 
Author: Richard Foote
  INET: richard.foote_at_bigpond.com

Fat City Network Services    -- 858-538-5051 http://www.fatcity.com
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Received on Fri Nov 07 2003 - 16:59:39 CST

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