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Re: Poor mans RAID10...

From: Howard J. Rogers <hjr_at_dizwell.com>
Date: Tue, 16 Mar 2004 07:40:51 +1100
Message-ID: <405614d5$0$3957$afc38c87@news.optusnet.com.au>

"Holger Baer" <holger.baer_at_science-computing.de> wrote in message news:c34o0d$gjo$1_at_news.BelWue.DE...
> Volker Hetzer wrote:
> > Hi!
> > I'm thinking of distributing one tablespace across several spindles
simply by
> > having one tablespace consisting of several datafiles.
> >
> > Does anybody know about how oracle (9i, Linux) actually allocates space
> > across the different datafiles?
> > Is one table always in one file or is the granularity the extent?
> >
> > Lots of Thanks!
> > Volker
>
> Hi Volker,
>
> extents are allocated in a round robin fashion, so with several datafiles,
> each datafile holds about the same number of extents.

That has not been true since 8i, if memory serves. It definitely isn't true in 9i.

The algorithm now appears to be to fill one disk up first, and then move on to the other. It is probably subtler than that, but it's not round robin.

Proof?

SQL> create tablespace test
  2 datafile 'c:\oracle\ora92\lx92\test01.dbf' size 5m,   3 'c:\oracle\ora92\lx92\test02.dbf' size 5m;

Tablespace created.

SQL> create table blah tablespace test
  2 as select * from emp;

Table created.

SQL> alter table blah allocate extent;

Table altered.

SQL> / <Repeat several times>

SQL> select file_id from dba_extents
  2 where segment_name='BLAH';

   FILE_ID


         7
         7
         7
         7
         7
         7
         7

7 rows selected.

This behaviour has always puzzled me, since I am fairly certain that in 8.0 and before the extents really *were* distributed alternately between the two datafiles (or round-robined in a multi-file tablespace).

Regards
HJR
>
> However, with regard to your subject: this is at best Raid 0 ;-)
>
> Cheers,
>
> Holger
Received on Mon Mar 15 2004 - 14:40:51 CST

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