oracle 常见的几种IO操作

Scattered Read, Sequential Read, and Direct Path Read

db file scattered read

出现原因：

(multiblock read into many discontinuous SGA buffers)

FTS(full table scan)全表扫描或IFFS (index fast full scan) 快速索引全扫描

从磁盘上读数据到内存中，一次I/O读取多个数据块，而数据块在内存中是分散分布并不是连续的。

优化方向：

I/0（Slow io）,sql调优(Poorly tuned SQL)。

db file sequential read 

出现原因：

数据块读入到相连的内存空间中(contiguous memory space)，而不是指所读取的数据块是连续的。

INDEX FULL SCAN/UNIQUE SCAN,此时出现”db file sequential read”

INDEX RANGE SCAN

direct read：

产生原因：

• The sorts are too large to fit in memory and some of the sort data is written out directly to disk. This data is later read back in, using direct reads.

• Parallel slaves are used for scanning data.

• The server process is processing buffers faster than the I/O system can return the buffers. This can indicate an overloaded I/O system.