Oracle Training Oracle Support Development Oracle Apps

 E-mail Us
 Oracle Articles
New Oracle Articles

 Oracle Training
 Oracle Tips

 Oracle Forum
 Class Catalog

 Remote DBA
 Oracle Tuning
 Emergency 911
 RAC Support
 Apps Support
 Oracle Support

 SQL Tuning

 Oracle UNIX
 Oracle Linux
 Remote s
 Remote plans
 Application Server

 Oracle Forms
 Oracle Portal
 App Upgrades
 SQL Server
 Oracle Concepts
 Software Support

 Remote S


 Consulting Staff
 Consulting Prices
 Help Wanted!


 Oracle Posters
 Oracle Books

 Oracle Scripts

Don Burleson Blog 








Oracle pushes low-cost Apple disks

Oracle is collaborating with the new Apple Xserv RAID devices, a low-cost data storage array with hardware RAID.  This disk array features “hot-swappable” disk “modules, and the device can hold up to 7 terabytes.  Life after Coffee’s Jon Emmons notes that the cost is $12k, but that is only $1.86 per gig.  The Apple pitch notes:

“the gigabyte-per-dollar ratio of Xserve RAID is the best in the world for Fibre Channel storage and trumps most SCSI storage solutions as well. Xserve RAID offers up to 7TB of high-performance storage at under $2 per gigabyte — a fraction of the cost of storage from Dell, HP, Sun or IBM.”

However, it’s the speed of the disk that counts, not the cost, and here is the spec sheet for the Xserv disks.  The first scary part is that each spindle has an 8 meg cache, a component that suggests highly variable access speed.

It turns-out that the disk spindles are 250 gig or 500 gig behemoths, and there are serious speed issues associated with super-large disks in Oracle databases.  If your whole database fits on a single spindle, the device can shake like an out-of-balance washing machine when the read-write heads get moving, especially if you have popular data files on the outer cylinders:

I always get suspicious when I don’t see average speed numbers (instead they use throughput numbers), so we have to wonder “what is the average access latency” of these spindles?

How fast is the Apple XServ RAID?

Oracle DBA's are interested in average access speed, not throughput numbers, and we have to do some math to guesstimate the average latency of the Apple XServ RAID disks.

Ironically, I could only find the average seek delay on an eBay ad: “Average seek time - 8.5 ms”.  We can compute the average rotational delay based on the spin speed (7,200 RPM), assuming that. on average, we have to wait for "half a revolution".  Hence, the average rotational latency is as follows:

7,200 RPM = 120 revolutions per second 

Hence, we have one revolution every 1/120th of a second = .00833 = 8.33 milliseconds per revolution.

Hence, the average rotational delay for this device (assuming that must wait for half a revolution) is 4.16 ms.

OK, now that we know the average seek and rotational latency, we can finally get the average latency, excluding data transmission time:

Average seek delay =             8.50 ms
Average Rotational delay =   4.16 ms
                                              12.66 ms 

Of course, the total average speed will be reduced by the on-board 8 meg data caches, but we still have the problem of minimizing the seek delay.  If we place frequently-accessed data files on the outer cylinders, the average seek delay will be far longer:


Oracle Training at Sea
oracle dba poster

Follow us on Twitter 
Oracle performance tuning software 
Oracle Linux poster


Burleson is the American Team

Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

Verify experience! Anyone considering using the services of an Oracle support expert should independently investigate their credentials and experience, and not rely on advertisements and self-proclaimed expertise. All legitimate Oracle experts publish their Oracle qualifications.

Errata?  Oracle technology is changing and we strive to update our BC Oracle support information.  If you find an error or have a suggestion for improving our content, we would appreciate your feedback.  Just  e-mail:  

and include the URL for the page.


Burleson Consulting

The Oracle of Database Support

Oracle Performance Tuning

Remote DBA Services


Copyright © 1996 -  2017

All rights reserved by Burleson

Oracle ® is the registered trademark of Oracle Corporation.

Remote Emergency Support provided by Conversational