Oracle FAQ Your Portal to the Oracle Knowledge Grid

Home -> Community -> Mailing Lists -> Oracle-L -> Re: Question on diff block sizes in DB

Re: Question on diff block sizes in DB

From: Jonathan Lewis <>
Date: Mon, 19 Apr 2004 19:29:53 +0100
Message-ID: <017b01c4263c$5084cbf0$7102a8c0@Primary>

Notes in-line.


Jonathan Lewis

The Co-operative Oracle Users' FAQ

April 2004 Iceland June 2004 UK - Optimising Oracle Seminar July 2004 USA West Coast, Optimising Oracle Seminar August 2004 Charlotte NC, Optimising Oracle Seminar September 2004 USA East Coast, Optimising Oracle Seminar September2004 UK - Optimising Oracle Seminar

On 04/18/2004 04:03:15 PM, Jonathan Lewis wrote:
> With a stripe size of 64k, That's a minimum of 4 discs
> kicked into action on a single read.

Is this bad? That means that every read larger then 64k will be done in parallel, and not sequentially.

> Now imagine you
> kick off a parallel tablescan: how many devices do you
> actually have for your 17GB, and how much I/O contention
> will a single parallel request cause on your system.

That is not necessarily true. Contention will be caused if and only if the channel between the RAID device and the RAM doesn't have sufficient throughput for transferring all that data, which is normally not the case, at least not with FC/AL type channels.

> If you are stuck with a 64K stripe size, I'd consider
> keeping your multi-block read size down to 64K in
> the hope of minimising contention..

You don't want to read in parallel? Why would that be good?

> But as a special
> can you guarantee that the boundary of a 32K Oracle data
> block will always synchronize with the boundary of a 64K
> stripe ?

On EMC Symmetrix I was told that it is possible to align raw devices in such a way. I have never managed an EMC array, so I cannot tell for certain.

> If you can't then perhaps one block out of every
> two will span two stripes, and therefore activate two
> disk drives. If you switch to 8K blocks then it's only
> one block in eight that would have the problem.

Well, that's an old wisdom which doesn't necessarily hold water with the latest Symmetrix and Shark boxes any more.

Please see the official ORACLE-L FAQ:

To unsubscribe send email to: put 'unsubscribe' in the subject line.
Archives are at
FAQ is at
Received on Mon Apr 19 2004 - 13:30:41 CDT

Original text of this message