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Re: BigFile Tablespaces

From: Tanel Põder <>
Date: Thu, 15 Jul 2004 22:22:00 +0300
Message-ID: <06ef01c46aa1$00cdc220$63a423d5@porgand>

> Trying to wade through the new 10G features. About bigfile tablespaces and
> ASM. If I understand it right, isn't this Oracle's way of trying to
> implement something along the lines of the S.A.M.E methodology?

We'll I think ASM's idea is much cooler than SAME, due additional flexibility and features (like redistributing IO etc.). But since ASM is very fresh and proprietary right now, I suggest to keep your redologs (and archivelogs) out from ASM on a decent filesystem or raw devices - that way you'll be able to fully recover from backup even when ASM instance hits the fan.

But if you wonder why bigfile tablespaces may contain only one datafile, then the reason is backwards compatibility and rowids. Rowids in bigfile tablespace have 32 bits reserved for data block number in given datafile file. In a smallfile tablespace rowid you have only 22 bits for specifying block number and the rest 10 bits were for stating in which datafile (relative to segment's tablespace) the data block resides. So, Oracle just "moved" 10 additional bits in rowid from one use to another, keeping the rowid sizes the same.

This means that, allowing some internal overhead, in a smallfile tablespace you can have 1022 datafiles, each consisting of up to 2**22-1 = 4 million blocks.
In a bigfile tablespaces, you can have only 1 datafile, consisting up to 2**32-1 = 4 billion blocks.

The total sizes add up the same, but for a multi-petabyte database it might not be reasonable to hold tens of thousands of files in database, for various reasons like opening or checkpointing the database. Also there is a database wide limit - you can't have more than 65533 files in an Oracle database (Once when I tested it, I did hit some other limit near 30000 files).


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Received on Thu Jul 15 2004 - 14:23:18 CDT

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