Tablespace size & initial extent size

Question:  I see a mismatch between my "size" parameter and the actual extent size in a tablespace:

CREATE SMALLFILE
TABLESPACE "TEST2"
NOLOGGING
DATAFILE '/u02/oradata/ORCLBD/TEST2.dbf'
   SIZE 5M
   EXTENT MANAGEMENT LOCAL
   UNIFORM SIZE 3K
   SEGMENT SPACE MANAGEMENT MANUAL;

My extents will be uniform and each one with 3K, but if 1 extent is a set of contiguous blocks, so at least I should have 8k for my extent (1 data block in my database is 8k).  But a query against dba_extents shows this as 16k for my initial extents.  Why do I see 16k instead of 3k as specified?

Answer:  The initial size of a tablespace is also governed by the MINEXTENTS parameter.  In this case, you did not specify in your create tablespace syntax, so the default in dba_tablespaces takes effect.  The minimum blocks for any table is 2 blocks, one block for the segment header, and one minimum data block. 

To examine the size of existing tables, I recommend the Oracle datafile and tablespace scripts to see details:

- Oracle tablespace and data file internals scripts

In your case, you specify a value that is less than the database blocksize (8k), but Oracle cannot allocate less than one block (plus another block for the segment header), so you see 16k (8k * 2) for your initial tablespace size.

Robert Freeman notes from his book "Easy Oracle Jumpstart":

An extent is an uninterrupted (or contiguous) allocation of blocks within a segment. Extents are assigned to a segment automatically by Oracle, so you will rarely deal directly with an extent, rather you will deal directly with its associated segment (table or index).

An extent must be on contiguous blocks within a single datafile, so an extent cannot span multiple Oracle datafiles. Oracle will allocate the size of the extents based on the type of tablespace.

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