Oraclenbsp;Internal之Blocknbsp;Cleanout

Oracle Internal之Block Cleanout 1

为什么会有Block Cleanouts? Oracle server在执行一个交易

(transaction)时会保存交易数据在数据块(data block)本身，并且一个交

易有可能修改大量的数据块,因此交易需要清理这些在数据块自己上的交易

数据当一个交易完成commit。
那些东西需要cleaned out?
  1. Row Locks
  2. ITL Entry:
          - Commit flags
          - Free Space Credit/Commit SCN

Deferred Block Cleanout:
    A transaction commit was a simple update to the relevant undo

segment header.
    The data block cleanout was left to the next reader of the

modified block
Fast Block Cleanout: This is the default action.
    Modified blocks are 'memorized' by a transaction. On

transaction commit, cleanout is attempted on those blocks in

memory(up to a limit 20 blocks). For any blocks above this

threshold, or aged out of memory, or currently pinned by another

process deferred block cleanout will be still used.


Oracle Internal之Block Cleanout: Commit Cleanout

一个交易commit后需要做fast block cleanout，但一个交易有可能更新多

个数据块，因此这个交易必须要记得那些block做过修改。 Oracle使用了一

个叫做Block List State Object来完成这个记住这些修改过的数据块。
这个BL State Object包含最多20不同的数据块的条目，每个BL State

Object条目包含如下信息：
  . 当前TX的Savepoint number
  . 当前TX的ITL索引
  . 相关数据块的块头的指针
当修改一个数据块时，TX将这个块的如上信息加入到BL State Object。
The maximum number of blocks "remembered" is effectively 10% of

current #buffer rounded to the nearest multiple of 2