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 Block Access Modes and Buffer States
Oracle Tips by Burleson Consulting

Oracle 11g Grid & Real Application Clusters by Rampant TechPress is written by four of the top Oracle database experts (Steve Karam, Bryan Jones, Mike Ault and Madhu Tumma).  The following is an excerpt from the book.

The buffer state indicates the status of a buffer in the local cache of an instance. Information about the buffer state can be seen in the dynamic performance view v$bh. The buffer state of a block relates to the access mode of the block. For example, if a buffer state is in exclusive current (XCUR) state, it indicates that the instance owns the resource in exclusive mode.

 

To see a buffer’s state, query the STATUS column of the v$bh dynamic performance view. The v$bh view provides information about the block access mode and their buffer state names as follows:

  • Buffer state ‘CR’ - indicates that the block access mode is N (null). It means the owning instance can perform a consistent read of the block, if the instance holds an older version of the data.

  • Buffer state ‘SCUR’ - indicates that the block access mode is S (shared). It means the instance has shared access to the block and can only perform reads.

  • Buffer state ‘XCUR’ - indicates that access mode is X (exclusive). It means the instance has exclusive access to the block and can modify it.

  • Buffer state ‘PI’  - indicates that block access mode is N (null). It means that the instance has made changes to the block but retains copies of past images.

BLOCK ACCESS MODE

BUFFER STATE NAME

DESCRIPTION

X

XCUR

Instance has exclusive access to the block and therefore can modify the block

S

SCUR

Instance has shared access to the block and can only perform reads

NULL

CR

Contains an older version of the data. Can perform consistent read

 --

PI

Past Image Exist  (useful for recovery)

Table 2.3:  Buffer States Shown in v$bh View

 

Only the SCUR and PI buffer states are real application cluster-specific. There can be only one copy of any block buffered in the XCUR state at any time. To perform modifications on a block, a process must assign an XCUR buffer state to the buffer containing the data block.

 

For example, if another instance requests a read access to the most current version of the same block, then Oracle changes the access mode from exclusive to shared, sends the current read version of the block to the requesting instance, and keeps a PI buffer if the buffer contained a dirty block.

 

At this point, the first instance has the current block and the requesting instance also has the current block in shared mode. Therefore, the role of the resource becomes global. There can be multiple shared current (SCUR) versions of this block cached throughout the cluster database at any given point of time.

     

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