RE: New 270.

John,

My experience has been that the OS writes new data to the drive with the
least per cent utilization, until such time as all of the drives in that
ASP are equalized.  Because of this, you need to be careful with empty
drives.  If all new writes were random adds to DB files, then perhaps
there were not be much impact.  But suppose you added a new 8.5 disk
drive to a system at 80 per cent disk utilization.  Then you restore a 6
GB library with full of database files.  All of those database files
will be restored to that one drive and any application using that
database would beat the living daylights out of that single drive.  The
same thing would occur if you did a copy of your largest database file;
it would all end up on that one drive.

If you had a mixture of 8.5 GB and 17 GB drives, then the system would
spread the data until all drives were at an equal per cent storage
utilization.  So you could expect your 17 GB drives to have twice as
much stuff as the 8.5 GB drives and thus twice the activity (assuming
the scatter loading is effective).  This would be the reasoning for not
having an ASP with a wide discrepancy in disk sizes.

The ASPBAL commands are pretty much required when adding new drives to
an ASP to avoid the activity imbalance.  Purists (or traditionalists)
may still do a system reload under these circumstances.

I've never seen an official explanation from IBM on their scatterloading
methodology.  I hope someone can find one.

Regards,
Andy Nolen-Parkhouse

> -----Original Message-----
> From: midrange-l-admin@midrange.com
[mailto:midrange-l-admin@midrange.com]
> On Behalf Of John Ross
> Sent: Monday, June 03, 2002 10:56 AM
> To: midrange-l@midrange.com
> Subject: RE: New 270.
>
> I was told last week by a business partner, not to go over double the
size
> of your smallest dive in your system, so if you have a 4 do not go
over an
> 8.  He tried to explain why but I really did not understand it.
Something
> about it would get more reads on the smaller drive.
>
> Would it matter how you let the system spread the data? If you just
put
> another drive in and do not force it to spread the data on the system,
it
> seams like the new data would be put on the new drive and might get
more
> reads then older data, at least for a while. If you are letting the
system
> spread the data (Not forcing it) does it only put writes on the new
drive
> or will it force data that it should only be reading to be moved. Does
the
> OS try to stay on the first drive(s)? (I think in my 38 days I heard
it
> did) IF so with 8's it might spread over 2 drives with 17's it could
all
> end up on the first drive. And it seams the OS would be used a lot. I
have
> used WRKDSKSTS for 15 min and the number 1 disk is 1st or second for
> percent busy, but only by 1%.
>
> Is there a link anywhere that explains how IBM spreads the Data?
>
> John Ross
> www.opensource400.org
>
>
> > > Subject: Re: New 270.
> > >
> > > Last year, I made the move from 6- 8 GB drives to 6-17 GB, and
> >performance
> > > suffered considerably.
> > >
> > > Al
>
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