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Home -> Community -> Mailing Lists -> Oracle-L -> RE: How much overhead for datafile headers?
Hi Steve
I've been told to add an extra 1M when creating datafiles on raw devices. is this too much overhead ?
During testing i've "dd" to a raw device datafile file in the hope of corrupting it. I had to "dd" about 4 - 5Mb before the DB wouldn't function. I assumed it was overwritting a very large header on the data file, can you shed a some light on this ?
Thanks
Steve Gough
Steve Adams
<steve.adams_at_ixo To: Multiple recipients of list ORACLE-L
ra.com.au> <ORACLE-L_at_fatcity.com>
Sent by: cc:
root_at_fatcity.com Subject: RE: How much overhead for datafile headers?
25/05/00 11:54
Please respond
to ORACLE-L
Hi Steve,
This is not a Veritas issue.
Your datafile header is just one block.
It is the bitmap for local space management that has messed you up.
The bitmap is always 64K, regardless of the database block size.
So, for locally managed tablespaces you always need to allow
64K more for "headers" than you might otherwise have done.
Regards,
Steve Adams
http://www.ixora.com.au/ http://www.oreilly.com/catalog/orinternals/ http://www.christianity.net.au/ -----Original Message----- From: Steve Orr [SMTP:sorr_at_arzoo.com] Sent: Thursday, May 25, 2000 6:06 AM To: Multiple recipients of list ORACLE-L Subject: How much overhead for datafile headers?
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:
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
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