RE: disk full guideline

["Al Barsa, Jr." <barsa2@xxxxxxx>]

At 11:59 AM 2/4/99 -0600, you wrote:
>Here is the text of a PMR that I just sent to IBM.  Any thoughts or
>comments?
>
>Problem Description:
>What is the rationale behind the 80% disk full guideline? We consistently
>run, without any apparent degradation, in the 90% range, as well as we have
>at the 80% level. During certain periods, the percent full approaches 96%,
>with no apparent degradation.
>
>We have 170 d/t6607 and 30 d/t6713 raided, at a usable 848 gig. Your
>guideline on 20% free equals 160 gig of dasd. Is this a reasonable
>guideline? The performance graphs show a difference of 2-3% between average
>and maximum.
>
>The holder of the purse is questioning the necessity of buying additional
>dasd because of an arbitrary guideline. Please provide me with some
>reasonable method of determining what is appropriate.
>
The 20% guideline is one that has been developed after many years of
experience.  I agree with you that when you start to get *LOTS of DASD, the
number may go down, but also as you start to get *LOTS of DASD, you may
well have more activity on the system.

Part of the 20% rule is that the system looks for places on DASD to page
memory out to, and if it can't find big clumps (a technical term), the
write to DASD causes excessive delay.  

IMHO, you should look at the % busy for the arms (from WRKDSKSTS).  If you
run consistently about 20% busy, or spike over 40% for long periods (except
when save/restore is running), you need more DASD.

Al




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Al Barsa, Jr. - Account for Midrange-L
Barsa Consulting, LLC.  
400 > 390

Phone:  914-251-1234
Fax:    914-251-9406
http://www.barsaconsulting.com
http://www.taatool.com

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