Oracle Internal之Block Cleanout

谯秀洁

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?
<wbr> 1. Row Locks<br><wbr> 2. ITL Entry:<br><wbr><wbr><wbr><wbr><wbr>- Commit flags<br><wbr><wbr><wbr><wbr><wbr>- Free Space Credit/Commit SCN<br><br>Deferred Block Cleanout:<br><wbr><wbr> A transaction commit was asimple update to the relevant undo<br><br>segment header.<br><wbr><wbr> The data block cleanout wasleft to the next reader of the<br><br>modified block<br>Fast Block Cleanout: This is the default action.<br><wbr><wbr> Modified blocks are'memorized' by a transaction. On<br><br>transaction commit, cleanout is attempted on those blocks in<br><br>memory(up to a limit 20 blocks). For any blocks above this<br><br>threshold, or aged out of memory, or currently pinned byanother<br><br>process deferred block cleanout will be still used.<br><br><br>Oracle Internal之Block Cleanout: Commit Cleanout<br><br>一个交易commit后需要做fast block cleanout，但一个交易有可能更新多<br><br>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<br><br>个叫做Block List State Object来完成这个记住这些修改过的数据块。<br>这个BL State Object包含最多20不同的数据块的条目，每个BL State<br><br>Object条目包含如下信息：<br><wbr>. 当前TX的Savepoint number<br><wbr>. 当前TX的ITL索引<br><wbr>. 相关数据块的块头的指针<br>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。<br>The maximum number of blocks "remembered" is effectively 10%of<br><br>current #buffer rounded to the nearest multiple of 2</wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>