When we insert a rows into a data block, Oracle has the choice of re-using a data block from the freelist or grabbing a "brand new" empty data block, extending the table into the tablespace and raising the high water mark (HWM), usually in chunks of five data blocks (as of Oracle8i).

When a table is truncated, coalesced or moved, we can see free data blocks above the high-water mark, but usually all data blocks inside a segment are below the HWM, with some logically full blocks (off the freelist) and the others available for rows.

There are two insert options "direct path insert" and the APPEND hint that effect the allocation of blocks to tables.

• Direct path inserts - These are performed with blocks above the table high water mark (i.e. no use of table freelists).
   
• Append - Append tells Oracle to extend the table and grab new, empty data blocks for the insert.

The issue of parallel inserts has a serialization issue with Oracle's TM locks.  Mark Bobak has some excellent comments on TM locking during table DDL:

As to the TM enqueue, well, any DML or DDL will take a TM enqueue. This is to protect the object's data dictionary definition. Conventional DML will take a TM enqueue in shared mode.

DDL takes a TM enqueue in exclusive mode. So, a table that's undergoing DML cannot have DDL executed. (The exclusive mode required for DDL is not compatible with the shared mode
the DML session already holds.) However, other conventional DML will be able to proceed, cause a shared mode enqueue is compatible with the shared mode already held.

In this way, Oracle allows concurrent DML, while preventing DDL on tables undergoing DML. Now, the direct-path INSERT is a special case. A direct-path INSERT takes a TM enqueue in exclusive mode, the same as DDL does. This prevents all other DML from occurring.

Honestly, I don't know enough about the details of the mechanism to clearly explain why this is required. I would speculate it's because otherwise, it would be possible to have direct-path INSERT modifying the HWM, and then (if it were permitted) conventional DML could fill all the blocks on the freelists, causing that session to want to raise the HWM.

That would be a problem, cause you'd have two different sessions wanting to raise the HWM at the same time. So, seems to me, you either serialize on the TM enqueue, or, you'd serialize on ST enqueue. (ST=space management transaction enqueue).
 

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