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Oracle Data Block Fundamentals

The following gives insight into Data block fundamentals.
  1. Freelist: Used to manage free data blocks
  2. High water mark: Used to track the last data block with row data
  3. PCTFREE: Determines when to perform a freelist un-link
  4. PCTUSED: Determines when to perform a freelist re-link
  5. Extents table: Used to manage new object extents

Data Blocks

Oracle manages the storage space in the datafiles of a database in units called data blocks. A data block is the smallest unit of data used by a database.
In contrast, at the physical, operating system level, all data is stored in bytes and each operating system has a block size. Oracle requests data in multiples of Oracle data blocks, not operating system blocks.
The standard block size is specified by the initialization parameter DB_BLOCK_SIZE. In addition, you can specify of up to five nonstandard block sizes. The data block sizes should be a multiple of the operating system's block size within the maximum limit to avoid unnecessary I/O. Oracle data blocks are the smallest units of storage that Oracle can use or allocate.