multiblock vs direct path read on 10gR2 aix (related: iostat/awr stat diff)

  • From: Jeremy Schneider <jeremy.schneider@xxxxxxxxxxxxxx>
  • To: joshuasingham@xxxxxxxxx
  • Date: Thu, 4 Aug 2011 09:43:51 -0500

Interesting.  I have a different question related to the same issue of I/O
on 10gR2 AIX. I wonder if anyone else on this platform can tell me whether
you've seen anything similar.

I have observed about a 10x difference in throughput for direct-path reads
vs multi-block reads of the same size.  In other words, as soon as a query
switches from multiblock to direct path, it suddenly does I/O literally 10
times faster.  I expect direct path to be faster (bypassing buffer cache
overhead and all), but this seems a bit extreme!  I'm curious about others'
experiences with direct path reads - has anyone else seen this big of a
speed difference between direct path and multiblock?

At one point, full tablescans were pulling table data at about 16MB/s with
direct path and about 180MB/s with direct path.  I haven't started digging
yet, but does anyone know if these two methods use the same system I/O
call?  If so, that might indicate something in the database accounting for
the difference, otherwise it could be a combination of DB and OS.  At the
storage tier, these tests were reading the exact same blocks.  I alternated
between them several times to make sure I wasn't just seeing cache

Also, I was calculating these throughput number from the wait events... and
I didn't see any wait events related to contention.  All the wait time was
"db scattered read" or "direct path read" - both reading 32 x 16k blocks per


On Wed, Aug 3, 2011 at 9:59 AM, joshuasingham <joshuasingham@xxxxxxxxx>wrote:

> Hi ,
> I been looking at some ( on AIX 64 bit Awr report File IO Stats
> and can see that the Av Rd(ms) for some of the files are consistently above
> 50 to 60 ms over a 15min interval but when I reported this storage guy he
> mention and sent me the Iostat to prove that the service time in the same
> timeline is average of 20ms for the disk involve . What can cause the
> difference between the AWR and iostat readings
> thanks

+1 312-725-9249

Jeremy Schneider

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