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Home -> Community -> Mailing Lists -> Oracle-L -> Re: How much overhead for datafile headers?
Steve,
There *should* be a small manual shipped with your software... but I also think the os specific docs are on either metalink or technet (try metalink first)
I'm taking the internals class this week. We just did data internals... no mention of a difference in the number of header blocks, but I can try to check with him tomorrow morning.
Rachel
>From: "Steve Orr" <sorr_at_arzoo.com>
>Reply-To: ORACLE-L_at_fatcity.com
>To: Multiple recipients of list ORACLE-L <ORACLE-L_at_fatcity.com>
>Subject: How much overhead for datafile headers?
>Date: Wed, 24 May 2000 12:05:36 -0800
>
>Is there a way to determine how much overhead is being used for a datafile
>header? From an X$ table?
>
>Here's what I used to "know:" 1) for file systems you need to allocate one
>database block for datafile overhead; 2) for raw the overhead is two
>db_blocks. 3) Other considerations for volume managers...
>
>Here's the story...
>I had a datafile sized to 1GB + 2 db_blocks (16KB each) with the idea that
>it could contain 8 equally sized extents of 128MB in a locally managed
>tablespace. Then I "discovered" that the actual size of the datafile
>overhead was 4 db_blocks so there wasn't any room for the 8th extent.
>:-(The first extent begins at block 5.) The DBA_FREE_SPACE view does not
>even list the tablespace, meaning it is full even though there is 131,040KB
>of unused space in the datafile. What a waste! Now I have a slew of
>datafiles to extend by 64KB.
>
>This was on a Veritas volume which probably means that the overhead for
>datafiles on Veritas is 4 db_blocks. The overhead on our non-Veritas files
>is only 1 db_block as expected. Supposedly you should be able to find this
>out from the platform-specific documentation but I didn't find anything in
>the Oracle or Veritas manuals which addresses datafile overhead
>requirements.
>
>
>Here's what the Concepts manual says:
>------------------------------------
>"Oracle creates a datafile for a tablespace by allocating the specified
>amount of disk space plus the overhead required for the file header...
>
>Additional Information: For information on the amount of space
>required for the file header of datafiles on your operating system,
>see your Oracle operating system specific documentation."
>
>
>Anyone have an idea where to find this info?
>
>
>Steve Orr
>
>--
>Author: Steve Orr
> INET: sorr_at_arzoo.com
>
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