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Home -> Community -> Usenet -> c.d.o.server -> Re: DB_BLOCK_SIZE on DIGITAL ALPHA
Oracle uses DB_BLOCK as the smallest unit to read/write the
information from the disk/memory. The DB_BLOCK_SIZE is chosen
at the time of database creation, and can not be changed thereafter.
All the datafiles created in the database are formatted by Oracle
based on this value of DB_BLOCK_SIZE. So, the change of DB_BLOCK_SIZE
will render already existing datafiles unusable because of mismatch
of DB_BLOCK_SIZE and dafa-file format, that is the reason why once the
dabase has been created you can not change the DB_BLOCK_SIZE. The only
way to acheive it would be to recreate the database with new DB_BOLCK_SIZE
and then import the objects from an export file.
The segments like tables,indexes,rollback segments,temporary segments require physical storage space on the hard disk. These objects when created are allocated INITIAL EXTENT, MINIMUM EXTENTS. Later when this the INITIAL EXTENT and other MINIMUM EXTENTS get filled Oracle has to allocate disk space on the disk based on NEXT EXTENTS.
What is an EXTENT?
An Extent is a contiguous set of Oracle Blocks with Block Size equal to
DB_BLOCK_SIZE on the disk, which are allocated for a particular segment.
(The segment could be a table, an index, a rollback segment space or a
temporary segment).
Just a note for clarification purposes: One extent belongs to one and only one datafile and one and only one object(segment) and it also belongs to one and only one tablespace.
Hope this clears you doubts and this stuff is helpful to you,
Manoj Jain
Oracle Certified Professional - DBA
Chauncey Certified Oracle7 DBA
On Thu, 06 Aug 1998 12:20:19 +0100, Caroline Keay <c.keay_at_cranfield.ac.uk> wrote:
> I have just returned from a DBA course where they emphasised
> the importance of setting the right DB_BLOCK_SIZE in
> relation to the operating systems block size. They said that
> most PC, NT and VAX systems have a block size of 512 bytes, but
> unix systems have a block size of 8k which can be accessed in 1k
> fragments. Before I go messing with my database I wanted to
> make sure what the block size is on an DIGITAL ALPHA UNIX system
> running OSF1. I can't find anything in the documentation and
> don't know where to look and nor does my Unix system administrator.
> Can anyone help.
>
>
> --
> * Caroline Keay ! SSLRC *
> * work: c.keay_at_cranfield.ac.uk ! Cranfield University *
> * home: caroline_at_keay.demon.co.uk ! Silsoe, U.K. *
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