Buffer Modes and States and I/O

Buffer States

The database uses internal algorithms to manage buffers in the cache.

A buffer can be in any of the following mutually exclusive states:

• Unused

  The buffer is available for use because it has never been used or is currently unused. This type of buffer is the easiest for the database to use.

• Clean

  This buffer was used earlier and now contains a read-consistent version of a block as of a point in time. The block contains data but is "clean" so it does not need to be checkpointed. The database can pin the block and reuse it.

• Dirty

  The buffer contain modified data that has not yet been written to disk. The database must checkpoint the block before reusing it.

Every buffer has an access mode: pinned or free (unpinned). A buffer is "pinned" in the cache so that it does not age out of memory while a user session accesses it. 

 

Buffer Modes

When a client requests data, Oracle Database retrieves buffers from the database buffer cache in either current mode or consistent mode.

The modes differ as follows:

• Current mode

  A current mode get, also called a db block get, is a retrieval of a block as it currently appears in the buffer cache. For example, if an uncommitted transaction has updated two rows in a block, then a current mode get retrieves the block with these uncommitted rows. 

• Consistent mode
  A consistent read get is a retrieval of a read-consistent version of a block. This retrieval may use undo data. For example, if an uncommitted transaction has updated two rows in a block, and if a query in a separate session requests the block, then the database uses undo data to create a read-consistent version of this block (called a consistent read clone) that does not include the uncommitted updates. Typically, a query retrieves blocks in consistent mode.

Buffer I/O

A logical I/O, also known as a buffer I/O, refers to reads and writes of buffers in the buffer cache.

When a requested buffer is not found in memory, the database performs a physical I/O to copy the buffer from either the flash cache or disk into memory. The database then performs a logical I/O to read the cached buffer.