Re: Bitmap header and footer sizes

Following the email you sent to me off-line, I think it was probably note 109630.1 that I saw a few days ago.

I have now done a couple of experiments, and can confirm that there is no such
thing as a bitmap footer, and that the
note is irrelevant (I avoid saying rubbish because it may have been based on a
detail in implementation that has changed since, say, Oracle 8.1.3).

(It is something of a relief to be able to say this, as my book has an entire chapter on tablespaces, including an explanation of how locally managed tablespaces work, and it does not mention 'footers'.

The so-called 'bitmap footer' is probably nothing more than the bit of wasted tablespace left after all possible uniform extents have been catered for.

For example:
With an 8K Oracle block, create a tablespace of 1024M, with uniform size 128K - on a file system, the file will be 8K + 1024M - the first 8K being for the invisible header block.

Now look at dba_free_space, you get one free extent starting at block 9 i.e. the bitmap extends up to block 8 - 64K - which can be shown by dumping blocks to be of type
  "File Space Bitmap Block"

The size of the free extent will be:

    8191 * 128 * 1024 * 1024 bytes
which means that 64K has 'gone missing', but that 64K is too small for an extent of 128K, and that 64K is one extent-size minus 64K. Dumping blocks past the single free extent you will find that the 64K is identified as

    "CORRUPT"
not as
  "File Space Bitmap Block"

Similarly with 4M extents -
The one free extent will start at block 9 The available space will be 255 x 4MB
The 'lost space' will be 4M - 64K

With 128M extents
The one free extent will start at block 9 The available space will be 7 x 128MB
The 'lost space' will be 128M - 64K

There is NO bitmap footer - there is only wasted space (which can be reclaimed by
growing the file that extra 64K).

--
Jonathan Lewis
Yet another Oracle-related web site:  http://www.jlcomp.demon.co.uk

Practical Oracle 8i:  Building Efficient Databases

Publishers:  Addison-Wesley
See a first review at:
http://www.ixora.com.au/resources/index.htm#practical_8i
More reviews at: http://www.jlcomp.demon.co.uk/book_rev.html



Jonathan Lewis wrote in message
<979076308.18178.0.nnrp-08.9e984b29_at_news.demon.co.uk>...


>

>Notwithstandin Oracle notes to the contrary, I am not

>convinced that (in 8.1.5 at least) there is such a thing

>as a bitmap footer.

>