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Database Configuration and I/O Issues(2)
File Striping
关于ASM的。
大概是说,把文件放在磁盘的不同扇区,均衡读取。
Tuning FTS Operations
Investigate the need for full table scans.
Configure the DB_FILE_MULTIBLOCK_READ_COUNT参数
可以决定实例一次读的数据库块数,这也影响了CBO的执行
计划。举个例子,如果oracle块一个是8k,那这个参数设
成8,就一次读64k,但这也不oracle完全能决定的,还能
依赖底层的操作系统。例如如果此时操作系统容许I/O的上
限是一次32K,则这就不行了。所以这可平台有关。这个参
数设置可以在system级别与session级别的。
v$session_longops 可以看长时间的全表扫描。一般是大
于6秒的。
在系统的统计数据级别上,看全表扫描的数据。
select name,value from v$sysstat
where name like '%table scan%';
Checkpoints
1.Incremental checkpoints
CKPT updates the control file.
During a log switch CKPT updates the
control file and the data file headers.
2.Full checkpoints
CKPT updates the control file and the data
file headers.
DBWn writes out all buffers on the
checkpoint queue.
增量检查点是先不写数据文件,只更新日志,速度就很快
,但是如果一旦发生垮掉实例,重启的时候需要把数据都
写到数据文件,耗时。
Full Checkpoints全检查点也分为两种,一种是完全检查
点,一种是只写到该表空间的全检查点。
1.complete
alter system checkpoint;
2.Tablespace
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