Re: Does Oracle server try to allocate extents on five-block boundaries?

On Sat, 14 Jun 2003 17:54:55 +0300, Sergey Adamenko wrote:

> Hi!
>
> I'm reading "Performance and Tuning" Student Guide for 8i.
>
> In chapter 9 "Using Oracle Blocks Efficiently" said:
>
> | Larger extents can improve performance slightly because the Oracle
> | server can read one large extent from disk with fewer multiblock reads
> | than are required to read many small extents. To avoid partial multiblock
> | reads, set the extent size to a multiple of 5 *DB_FILE_MULTIBLOCK_READ_COUNT.
> | Multiply by five because the Oracle server tries to allocate extents on five-block boundaries.
> | By matching extent sizes to the I/O and space allocation sizes, the
> | performance cost of having many extents in a segment is minimized.
> | However, for a table that never has a full table scan operation, it makes no
> | difference in terms of query performance whether the table has one extent
> | or multiple extents.
>
> I woudered about mentioned algorithm of "...allocating extents on five-
> block boundaries". No mention about it in "Oracle Documentation
> Library, Release 8.1.7". Only good known principles of allocation for
> DMT with its "MINIMUM EXTENT" and LMT with its "UNIFORM SIZE" or
> "AUTOALLOCATE".
> Same results when searching in Goggles.
>
> My question is: is this principle discarded relating to 8.1.7 or I missing something?
>
> Thank you!
>
> Sergey Adamenko.

In a dictionary managed tablespace, if you don't specify "Minimum Extent", the default for "initial" is 5 blocks. The advice should read "always specify extent sizes in multiples of DB_FILE_MULTIBLOCK_READ_COUNT parameter". That advices is going into oblivion, together with the DMT. It's a bit mapped world now.

-- 
Mladen Gogala
Software is like sex, it is better when it is free.
Linus Torvalds