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Redo Wastage

原创 Linux操作系统 作者:ylw66 时间:2009-08-13 14:09:38 0 删除 编辑

LGWR commonly writes up to and including the current log buffer block, which is normally only partially full. The SGA variable containing the index into the log buffer for redo generation is then advanced to the beginning of the next log buffer block before LGWR releases the redo allocation latch prior to writing. In this way, some space is left unused in the last log block of most redo writes. The number of bytes skipped is accumulated in the redo wastage statistic. The following diagram illustrates redo wastage within the first few blocks of a sample Oracle redo log file.

LGWRlog buffer block写入log file时,通常log buffer block都不是满的。

SGA维护生成新的redo空间的Index的变量, 当LGWR释放redo allocation latch写之前,这个变量move到下一个log buffer block的开始(即SGAredo log buffer中为redo分配新空间,也是以block为单位的)

在这种模式下,大部分redo写的时候最后一个log block的一部分空间还没有被使用,这部分跳过的字节被统计为redo wastage

 

Redo wastage is not a problem. Indeed, it benefits performance by preserving LGWR's sequential I/O pattern. However, high redo wastage is a symptom of a very active LGWR, and that might be a problem. LGWR is expected to be highly active in many instances, but it can be overactive if the log buffer (more correctly, the _log_io_size) is too small, or if the commit rate is too high. If LGWR is overactive, it will place unnecessary load on the redo latches, and on the I/O subsystem. The operating system may also degrade LGWR's execution priority.

redo wastage 并不是问题。redo wastage有益于保持LGWR顺序I/O机制的写的性能

LGWR是顺序I/O方式将redo写入log file,在这种方式下批量写入,并不管redo wastage会提高性能)

但是高的redo wastageLGWR活跃的征兆,这有可能是个问题。在许多情况下我们希望LGWR活跃,但是如果log buffer比较小或者commit很频繁,就会造成LGWR过于活跃。如果LGWR过于活跃,就会造成在redo latchI/O subsystem上不必要的负荷

 

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