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How much overhead for datafile headers?

From: Steve Orr <sorr_at_arzoo.com>
Date: Wed, 24 May 2000 11:55:51 -0700
Message-Id: <10507.106567@fatcity.com>


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? Received on Wed May 24 2000 - 13:55:51 CDT

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