Re: direct read is much faster than scattered read, is it reasonable?
Date: Wed, 28 May 2008 11:15:20 -0500
Oh how I wished my reads were as "slow" as 1.8ms.
I don't think you can draw any conclusions from this. I would hypothesize that many of your reads are from the SAN cache. If scattered reads had a higher percentage that were read from disk, that "could" cause the results you are seeing.
Are you trying to solve a problem or is this just an academic exercise.
- Original Message ----- From: qihua wu To: oracle-l Sent: Wednesday, May 28, 2008 4:45 AM Subject: direct read is much faster than scattered read, is it reasonable?
I executed these sql on my database:
SQL> select time_waited_micro/total_waits/1000000 from v$system_event where event_id in (select event_id from v$event_name where name='direct path read');
SQL> select time_waited_micro/total_waits/1000000 from v$system_event where event_id in (select event_id from v$event_name where name='db file scattered read');
We could see: to access one block using scattererd read, it will take 1.8 ms, but when access one block using "direct path read", it only takes 0.5ms. How could direct path read much much quicker than scatterer read? Isn't the scattered read the fastest method to access data block? Our system uses SAN storage, and SAN could pre-fetch to speed-up the block access. But I think it pre-fetch will help both scattered read and direct path read, am I right?