Re: Detecting xxx fragmentation/corruption?

From: Peter Teoh <>
Date: Sun, 11 May 2008 22:38:17 +0800
Message-ID: <>

On Sat, May 10, 2008 at 9:55 AM, Daniel Fink <> wrote:
> To supplement Jared's response... Inline
> Peter Teoh wrote:
>> Unlike memory, where access time almost zero, accessing the disk is
>> much slower. So sometimes I thought it will be better to spread out
>> the data - thus enable simultaneous read by the different heads in the
>> disk, just like those RAID design. Ie, fragmentation via
>> distribution the blocks out in the disk can improve performance - can
>> such things happened?
> In terms of reading data in memory, access time may be almost zero, but the
> time required to read a block in memory is not thousands of times faster
> than reading it from disk. I don't recall the exact figure from a Cary
> Millsap presentation ("Why You Should Focus on LIOs"?)some years ago, but
> the difference was something like 42 times. If you add in time required to
> create a read consistent version, a single logical i/o could take much more
> time than a physical i/o.

I have yet to find Cary Millsap article, but this (by Don Burleson) is interesting:

One of the issues is the relatively high-cost of fetching an Oracle data block from disk. In theory RAM is 10,000 times faster than disk milliseconds vs. nanoseconds), but when you add-in the overhead of lock serialization and latches, a logical I/O might be less than a thousand times faster than a "physical" disk I/O.

And this one from AskTom specifically on LIO is interesting:

> --
> Daniel Fink

In general...I do agree that LIO involves lots of transaction handling, and CPU crunching...esp with Oracle read-consistency requirements.


Peter Teoh
Received on Sun May 11 2008 - 09:38:17 CDT

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