Dennis,
I would add one more thing to what Rob said. Your paging rate
should also be taken into account. If 100 "spindles" will handle your
peak paging rates without causing measurable page wait, I'd vote for
going the all-3390-9 route rather than the -3 approach.
And record one more vote that mixed sizes is a very bad idea (for
the reasons Rob described).
Marty
On Thu, Sep 30, 2010 at 4:32 AM, Rob van der Heij
<[email protected]> wrote:
> On Thu, Sep 30, 2010 at 12:53 AM, O'Brien, Dennis L
> <dennis.l.o'[email protected]> wrote:
>
> [ snip ]
> need them, so if I add another 32 GB of storage, my choice would be
> something like 212 3390-3's and 28 3390-9's, or 100 3390-9's. Which
> would be a better choice?
>
> As you indicated, z/VM spreads paging effectively to fill volumes with
> the same amount of data. This will fill your -3's quicker than the
> -9's. The risk is that at some point you filled the small ones
> completely and end up paging only with a subset of the subchannels.
> Mixing sizes is not recommended; depending on the actual numbers it
> may be a very bad idea (tm).
>
> Bottom line is to look at your performance monitor and determine
> whether users are unreasonably held back by paging and whether that
> troubles your users.
>
> Let's do the math: once you have filled 50% of your paging space (~
> 340 GB) each will hold ~1.4 GB. So the -3's are at 60% and the -9's
> are at 20%. You'd still have 240 packs more or less contribute in
> paging. When you convert to all -9, you only have 100 packs doing the
> paging. So in this case, 240 is more than 100 ;-) This is assuming
> modern DASD, not the true 3390-9 that were rotating slower so you
> could stuff more bytes on a track (and take 3 times as long to read
> and write the data).
>
> If you plotted the dates when you added page packs, you may be able to
> predict when you post again... ;-) and knowing that you will run out
> of the maximum number of CP owned volumes real soon... suppose you
> would go for 28 extra 3390-27 instead. Once you fill that space for
> 50%, your -3's are stuffed and you only have space on the big ones, so
> you run with only ~10% of your paging volumes (that counts as bad
> idea). Would you have started with all -27's, you now had about twice
> the number of page packs working for you.
>
> PS I'm not making up these examples. We did see a customer convert to
> just a few very big page devices. They were not happy.
>
> Rob
> --
> Rob van der Heij
> Velocity Software
> http://www.velocitysoftware.com/
>
