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分类: Oracle

2011-07-09 21:43:20

What’s the Point of Oracle Checkpoints?
Buffer Cache Basics

The buffer cache component structures are:

  • Buffers
    • Each buffer may hold an image of one data block at any one time
  • Buffer headers
    • Store metadata about contents of the buffers
    • Act as cache management structures
  • Buffer pools
    • Collection of buffers used for the same purpose and managed accordingly
  • Working set
    • All or part of a buffer pool
    • Assigned to a DBWn process
Buffer Management
  • Cached buffers managed by doubly linked lists:
  • REPL
    • Buffers containing block images being used
  • REPL-AUX
    • Buffers ready to be used for I/O or CR build
  • WRITE and CKPT-Q
    • Dirty Buffers requiring I/O
  • WRITE-AUX
    • Dirty Buffers with I/O in progress
  • Touch count is used to decide the initial insertion location in the REPL chain
  • AUX lists avoid wasteful scanning
Redo and the Buffer Cache
  • Block modification dirties the buffer containing the block image and generates redo
  • A buffer becomes dirty at a particular RBA which is a point in the redo stream
  • Redo written by LGWR makes the corresponding part of the redo log file “active”
  • Dirty block images written by DBWn makes the corresponding part of the redo log file “inactive”
  • Redo is always written prior to the corresponding block images
  • Size of active redo in the log file influences instance and crash recovery time
  • Trade-off between performance and recovery time
Buffer Cache I/O
  • Servers look for an available buffer on REPL-AUX then read a data block into selected buffer
    • Buffer gets moved from REPL-AUX to REPL
    • If block is modified, buffer is added to CKPT-Q
    • Servers move dirty buffers to WRITE during free buffer search
  • DBWn writes dirty buffer contents to database
    • Buffer gets moved from WRITE to WRITE-AUX
    • Once block written:
      • Buffer is moved back to REPL-AUX
      • Buffer taken off CKPT-Q
  • DBWn writes upon request
    • Make free buffers
    • Checkpoint
Parallel Query Checkpoint
  • Writes block images to the database for all dirty buffers belonging to objects accessed by the query from all instances
  • Statistics updated:
    • DBWR checkpoint buffers written
    • DBWR checkpoints
  • Caused by:
    • Parallel Query
    • Parallel Query component of PDML or PDDL
    • Mandatory for consistency
Log Switch Checkpoint
  • Writes the contents of “some” dirty buffers to the database
  • Statistics updated:
    • DBWR checkpoints
    • DBWR checkpoint buffers written
    • background checkpoints started
    • background checkpoints completed
  • Controlfile and datafile headers are updated
    • CHECKPOINT_CHANGE#
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