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RE: Fav. Urban Legend...Mem vs Disk

From: DENNIS WILLIAMS <DWILLIAMS_at_LIFETOUCH.COM>
Date: Wed, 20 Mar 2002 06:13:23 -0800
Message-ID: <F001.0042E51B.20020320061323@fatcity.com>


Connor - Cary Millsap presented the results of 10 trace files in a Hotsos seminar I attended. The ratio ranged from a high of 108.57 down to a low of 0.79. The point is that the ratio is nowhere near the oft-quoted 10,000. This means that logical I/Os are not insignificant. Even if physical I/O were eliminated (all blocks cached, 100% cache hit ratio), response time would not drop to zero. This is why the emphasis in tuning is shifting from simple ratios to examining wait times. If the most significant wait time is physical I/O, then changing that will improve overall performance. But if the most significant wait time lies in another area, then you may make significant improvements in physical I/O and still not improve overall performance. I certainly wouldn't claim to be an Oracle tuning expert, but I believe that the new ideas on tuning that are emerging provide a significant step forward in making Oracle tuning more of a logical process than a collection of rules of thumb.
Dennis Williams
DBA
Lifetouch, Inc.
dwilliams_at_lifetouch.com

-----Original Message-----
Sent: Wednesday, March 20, 2002 3:49 AM
To: Multiple recipients of list ORACLE-L

Some rudimentary testing on a laptop here (500Mhz, 512M RAM, typical single disk)

  1. visiting a single block via 4,000,000 logical IO's got me approx 35000 gets/sec
  2. repeated full table scans similar system got me approx 350 phys reads/sec

After this extensive, thorough and exhaustive exercise, I can definitely say that memory access versus disk access (as it pertains to Oracle) is 100 times faster on this machine in single user mode

I think we can generalise this to be the rule for all servers under all conditions :-)

Connor


Connor McDonald
http://www.oracledba.co.uk (mirrored at http://www.oradba.freeserve.co.uk)

"Some days you're the pigeon, some days you're the statue"



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Received on Wed Mar 20 2002 - 08:13:23 CST

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