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Re: ORACLE on Linux - IO bottleneck

From: Fabrizio Magni <fabrizio.magni_at_mycontinent.com>
Date: Thu, 09 Feb 2006 15:14:20 +0100
Message-ID: <43eb4e3c$0$13116$892e7fe2@authen.yellow.readfreenews.net>


Mladen Gogala wrote:
> On Wed, 08 Feb 2006 23:35:14 -0800, Wyvern wrote:
>

>> I´ve not understood very well the IO Scheduler function but I´ll read
>> more about it ...

>
> There isn't much to understand: process scheduler decides which process to
> execute first, I/O scheduler decides which one, among the several
> simultaneous I/O requests to the same device, should be fulfilled first.
> The need for scheduling arises only if you have multiple simultaneous
> requests for the same device most of the time, in which case you have a
> problem, with or without the scheduler. I addition to that, most of the
> smart SCSI controllers have their own schedulers. The effects of I/O
> scheduler are much overrated.
>

Hi Mladen,
actually the I/O scheduler(s) works even if the I/O request are not simultaneous.
typical is the merging of I/O for sequential reads or writes.

The advantages can be tremendous.

Here is an example (same hardware, same device):

time dd if=/dev/zero of=/dev/raw/raw3 bs=8k count=10000 10000+0 records in
10000+0 records out

real 0m41.063s
user 0m0.016s
sys 0m0.412s

time dd if=/dev/zero of=/u02/foo bs=8k count=10000 10000+0 records in
10000+0 records out

real    0m2.324s
user    0m0.005s
sys     0m0.536s

The second dd has its I/O merged.

Consider this controversial but the I/O scheduler eliminate the need to have oracle block size "aligned" to file system block size (even without direct I/O).

Regards

-- 
Fabrizio Magni

fabrizio.magni_at_mycontinent.com

replace mycontinent with europe
Received on Thu Feb 09 2006 - 08:14:20 CST

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