oracle 常見的幾種IO操作

Scattered Read, Sequential Read, and Direct Path Read

db?file?scattered?read

出現原因：

(multiblock read into many discontinuous SGA buffers)

FTS(full table scan)全表掃描或IFFS (index fast full scan) 快速索引全掃描

從磁盤上讀數據到內存中，一次I/O讀取多個數據塊，而數據塊在內存中是分散分布并不是連續的。

優化方向：

I/0（Slow io）,sql調優(Poorly tuned SQL)。

db?file?sequential?read

出現原因：

數據塊讀入到相連的內存空間中(contiguous memory space)，而不是指所讀取的數據塊是連續的。

INDEX FULL SCAN/UNIQUE SCAN,此時出現”db file sequential read”

INDEX RANGE SCAN

direct?read：

產生原因：

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.

Oracle

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