Re: Disk: Percent full a performance factor?

Yes BUT. Fragmentation will begin to create issues.

The reason % busy can climb when disks fill is that instead of just driving the car into the garage you must first remove the wheels the front and rear bumpers and put them into the garage. Then lower the drop top and tip it on it's side on a cart and slip it in the available space. Yes the trip to work tomorrow is gonna suck too as you have to undo this effort to drive it again.

So enough arms as Jim suggests helps with this but extra effort is still required it's just hidden by extra capacity. If that's OK in your shop good for you. (Really!) But if you're pushing the %busy to a point where those extra I/Os hit knee of curve then that's bad.

It makes a HUGE difference what sort of I/O you are doing. If it's causing the data to move about with sorting and index builds and reorgs and restores and copies then this will matter much more to you as fragmentation will be far worse with that activity.

On the flip side I have customers with IBM i 7.1 as a host partition with nothing but NWSSTG filling the disk to 95%. They have been that way for over a year and a half with no issue whatever because those things are written to their full size when built and never move. The I/O is just within them.


- Larry "DrFranken" Bolhuis

www.frankeni.com
www.iDevCloud.com
www.iInTheCloud.com

On 10/20/2014 2:53 PM, Jim Oberholtzer wrote:

Rob,

It depends on how many DASD devices there are. If there are say 40-50
devices then running up to 85% -- 88% might not be a problem, but with 6
devices, you're in for a hard time.

Generally Databases tell you the knee in the response time curve starts at
about 80% and steepens quickly but with IBM i, as long as the DASD device
busy percentages are OK and there is room for temporary indexes, then that
percent full you have to be concerned with goes up as well, so higher
percent full is OK.

--
Jim Oberholtzer
Chief Technical Architect
Agile Technology Architects


-----Original Message-----
From: MIDRANGE-L [mailto:midrange-l-bounces@xxxxxxxxxxxx] On Behalf Of
rob@xxxxxxxxx
Sent: Monday, October 20, 2014 1:29 PM
To: midrange-l@xxxxxxxxxxxx
Subject: Disk: Percent full a performance factor?

The old rule of thumb I had learned back on the S/36 was that you really
didn't want to go over 80% of disk space used. I had a manager (came from
the programmer pit and rose through the ranks) that wondered if that still
held true with the growth of disk. After all, 20% free of a whole gig of
disk space sure was a lot more than 20% of 300MB. I argued that object size
was significantly larger and that reorgs and whatnot still needed
significant portions of disk space.
Now that we're measuring disk in TB and MB is nought but a rounding error
does 80% still hold true? If so, why?


Rob Berendt
--
IBM Certified System Administrator - IBM i 6.1 Group Dekko Dept 1600 Mail
to: 2505 Dekko Drive
Garrett, IN 46738
Ship to: Dock 108
6928N 400E
Kendallville, IN 46755
http://www.dekko.com

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