ITEM: BM7584L

DB2 -How to flush the buffer pool?

Env:
  AIX 3.2.5
  DB2 V2.1

Desc:
  Questions on information returned by DB2 System monitor.
  In comparing buffer pool physical and logical reads, the
  logical reads are about 4-5:1 vs the physical.  Want to know
  how to 'flush' the buffer pool, in order to get 'accurate'
  timing information on the physical i/o.

Act:
  Explained that as far as I know, the only way to 'flush' the
  buffer pool is to stop/start the db.  By stopping the db, all
  addresses to the memory locations for the buffer pool are lost
  and the original buffer pool memory address is released back to
  the system for general use.  When the db is started, it
  will allocate a new memory address for the necessary resources
  (ie. buffer pool).

  Customer next asked about the relationship between physical and
  logical read from/in the buffer pool.  Explained that the 
  physical reads are the actually physical disk i/o to retrieve
  the data from disk and place it in the buffer pool.  The 
  physcial read will read a disk block (4k), and it will be
  placed in the buffer pool.  The logical reads are the application
  (DB2 in this case) retrieving the data from the buffer pool.
  An entire page (4k) is again read from the buffer pool, and
  and the application will then break this page into the desired
  logical records/rows.

  In addition, db2 has a prefetch paramter that will preread
  information and fill the buffer pool.  Customer indicated that
  that his prefetch is set to 16.  In this case, in addtion to the
  physical i/o that will return a 'disk' page to the buffer pool,
  the prefetch parameter will continue physical i/o until the 
  'next' 16 (database) pages have also been placed in the buffer 
  pool.



Support Line: DB2 -How to flush the buffer pool? ITEM: BM7584L

Dated: December 1996 Category: N/A

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