DB_BLOCK_SIZE, DB_nK_CACHE_SIZE and BLOCKSIZE

First I want to say about BLOCKSIZE cluase in oracle. The BLOCKSIZE cluase is used to specify the block size for the tablespace. If you don't specify this clause while tablespace creation then standard that is default blocksize is used which is specified by parameter DB_BLOCK_SIZE. For example, You have DB_BLOCK_SIZE set to 8K=8192 and you specified CREATE TABLESPACE command without any BLOCKSIZE clause then database use 8k blocksize for the specified tablespace.

The parameter DB_BLOCK_SIZE specifies the size of Oracle database blocks in bytes. The default value of this parameter is 8192 and value ranges between 2048 to 32768. But it must be multiple of physical block size at device level.

Now lets have an attention of the DB_nK_CACHE_SIZE parameters. Here n is variable and can be 2, 4, 8, 16, 32. That is DB_2K_CACHE_SIZE, DB_4K_CACHE_SIZE, DB_16K_CACHE_SIZE etc are available. Now question may come why we will set DB_nK_CACHE_SIZE? What advantage we will get. Before look for advantage (which will be discussed in another topic) let me why we need to set this parameter? This parameter will need to be set whenever I wish to make or want to make a tablespace with non-standard data block size.

Suppose your standard block size that block size set by DB_BLOCK_SIZE is 8K and you want to make a tablespace with block size 16K then at first you need to set DB_16K_CACHE_SIZE and then you need to create tablespace with BLOCKSIZE clause specifying 16K.

Here is an example which shows database standard block size is 8k and you have made an tablespace with 16k blocksize.

Example:
------------------------------
SQL> show parameter db_block_size
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
db_block_size integer 8192

ALTER SYSTEM SET DB_16K_CACHE_SIZE=200M SCOPE=BOTH;
ALTER SYSTEM SET DB_CREATE_FILE_DEST='/oradata2';
CREATE TABLESPACE TEST BLOCKSIZE 16K;

Remember You can set this parameter only when DB_BLOCK_SIZE has a value other than nK. For example, if DB_BLOCK_SIZE=8192, then it is illegal to specify the parameter DB_8K_CACHE_SIZE Because the size for the 8 KB block cache is already specified by DB_CACHE_SIZE.
Related Documents

Choose an Optimal Block Size in Oracle