On Monday December 6, Marcy Cortes arranged the electrons to say: >>In my SHARE user experiences presentation I usually warn people against even >>answering that question. >>Stephen has a good approach to an answer. >> >>My boss thinks I'm a smart xxx when I turn it around and ask him >>"How many = rocks can you carry?" >>Obviously, not all rocks are created equally. >> >>I've had 2 z9 IFLs running about 100 servers (test/dev). >>I've had 6 z10 IFLs running 12 servers (prod). >> >>The answer is probably between 2 and 50 (although now with a 196 it's probably more like >>between 2 and 150). >>Is that helpful? Yeah, I thought not. >>Marcy
I really think answering the question or not depends on why the question is being asked in the first place. I'm getting ready to put an application in production that is going to require somewhere around 25 zLinux guests. If I just use my ratio of 15:1 for this, then I don't need to measure performance, I can just say that 2 IFLs will run the application just fine, with capacity for exactly 5 more servers left over, right? Wrong answer! When I go buy IFLs to support this, I'll buy capacity based on what we've seen in test, what the vendor says about their application, and a few other fact based estimates, and then make darn sure I have enough IFLs so that the thing runs as fast as greased lightning. So, even though my cost model says 15:1 for prod servers to IFLs, I don't pay attention to that when I need to buy capacity for a specific application. This is primarily because of the impact to the bank and our customers if I'm wrong. I'd rather be conservative, and have too much capacity than not enough. After the application is production, if I aimed too high, I'm not concerned - more stuff will come along behind this application to fill up the capacity. Of course, with budgets and other constraints, I can't just go crazy and buy 2X or 3X what I think I might need - thus the time spent estimating capacity needs. On the other hand, if I'm asking the question to determine a cost model or business case, things are different. If I look across a large enough population of servers, and I'm interested in building a charge back model, being 100% right really doesn't matter. With charge back's, if I recover somewhere between 80% and 120% of the real costs, I've done well. I might over recover zLinux costs, by my counterpart over our network engineering area might under recover the network costs. To build a charge back model, I have to make some assumptions and some educated guesses. Thus, my rule of thumb is 15:1 for prod servers:IFLs and 30:1 for test. So far, at least in my world, reality seems to line up closely enough to this that I'm happy sticking with these numbers. The actual ratio will also go up or down over time as I add virtual servers, use up spare capacity, buy more capacity, shut down virtual servers (ok, so that really never happens) etc. So, I'm not just looking for a point in time ratio - but more of what the ratio looks like over 6 to 12 months. I also totally agree that not all zLinux servers are created equal. I could probably find 3-4 servers that alone could consume 1 IFL, and I probably could find 60 other servers that together wouldn't fill up 1 IFL. But, when I look at my whole environment over a 6-12 month period of time, these tend to cancel each other out. So, Marcy, I agree - if someone comes to me and says "My application is going to need 50 virtual servers. How many IFLs do I need to run it?" I'm going to say "Probably somewhere between 1 and 25." But, if someone comes to me and asks me what my charge back rate is going to be for a zLinux server in 2011, I'll base my answer on my educated guess of 15:1 and 30:1. For everyone else - please, don't send me any flame mail if your ratios end up 5:1 or 50:1! (As always, your mileage may vary!) Randy ---------------------------------------------------------------------- For LINUX-390 subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO LINUX-390 or visit http://www.marist.edu/htbin/wlvindex?LINUX-390 ---------------------------------------------------------------------- For more information on Linux on System z, visit http://wiki.linuxvm.org/
