RE: Async IO on Linux

  • From: "Hostetter, Jay M" <JHostetter@xxxxxxxxxxxxxxxxxxxx>
  • To: <Oracle-L@xxxxxxxxxxxxx>
  • Date: Fri, 22 Sep 2006 10:40:57 -0400

I did the relink based on note 365416.1 (Oracle doesn't report the io
activity to the OS).

When I change filesystemio_options to setall, I see that Direct IO is
being used, by doing the following:
strace -fo temp.txt sqlplus
Online then offline a tablespace
Then grep -i o_direct temp.txt

I'll have to come up with some other scenarios for testing.

Thanks for the responses,

-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx
[mailto:oracle-l-bounce@xxxxxxxxxxxxx] On Behalf Of Kevin Closson
Sent: Thursday, September 21, 2006 3:58 PM
To: Oracle-L@xxxxxxxxxxxxx
Subject: RE: Async IO on Linux

...Oracle 10gR2 does not require a recompile for async on Linux BTW, it
is the first release where it's just in there. your question though... I have to first ask, are you using direct
I/O as well?
It doesn't smell like it to me... if you are not using direct I/O, async
I/O wont get you much really because all the writes on a buffered
filesystem are in essence async because the only file where writes are
synced is redo (open O_SYNC).

The import should push DBWR to show some benefit, but mostly with Direct
I/O. As for queries? Well, foreground processes do not use async I/O
reads for queries. Foregrounds use readv (blocking) for scattered reads,
and pread (blocking) for direct path reads and db file sequential reads.
PQO on the other hand, does indeed scan with async I/O keeping 4
sequential reads in flight at all times per slave (if there is enough
But, don't be confused, if the query plan is an index range scan, PQO
will use readv (blocking scattered reads)...

the short answer is asnyc only relates to certain things and you have to
test those to see a performance delta...oh, and you have to have a
bottlenect that async addresses such as free buffer waits ...

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