воскресенье, 15 ноября 2015 г.

Virtual DataCenter Design. Host Sizing. 1


A lot of times I see two contradictory approaches on how to include HyperThreading to sizing. You can just ignore HT at all, and calculate CPU power based on “real” cores, or you can double CPU power because you see twice as much logical CPUs. Unfortunately they’re both wrong. In general you can safely assume HT as +25% CPU, but with some ifs. The main IF is that HT makes most of the efficiency on workloads with a lots of small VMs. Let’s assume we have a dual-way host with 12-core CPUs. That means 24 “real” cores, or 30 “effective” cores. But 24 vCPUs would still be the widest VM you should place on that host.

1 or 2 or 4

Should we choose 2-way or 4-way servers for virtualization? Pretty common problem. So how to solve it?

Target workload is the answer 100% of the cases, and fault domain size. Fault domain in this case is one host. Let’s assume we have a target workload of 200 VMs that can be handled by 10 dual-way hosts with 256GB of RAM each. It’s not a rocket science to deduct that 5 4-way hosts with 512 GB of RAM can handle this workload as well. But! In case of HA event we’ll have double number of VMs affected with services, double number of VMs to restart, increased load on SAN. We have 20% of resources gone from cluster instead of 10% for dual-way servers. And that’ not all, for the planned maintenance we have to evacuate twice as much VMs which leads us to 2-2,5 times more evacuation time.

Latest improvements in CPU design bring us unprecedented number of cores per single socket, making 2-way vs 4-way quarrel even more complicated. Why the hell not 1-way servers with 16-18 cores CPU?

Conclusion. 4-way servers should be used in such cases as:
- Need to use Xeon E7 CPUs instead of E5. But still we can use 4 way servers with 2 sockets filled.
- Need for ultra-wide VMs size of full 2-way server or more, without possibility of application level clustering to split the load.

CPU choice

How to choose CPU in the family – that’s the question. You can try to stick your finger in the sky, use the coin or even a prophecy. But still most of the choices are made without real thinking. Let’s take a view on some basic aspects.
- NUMA. All the current x86 CPUs have the NUMA architecture. So each CPU has it’s own memory instead of globally shared. You have to access another CPU’s memory through the owner which is slower. But that’s not actually the problem, it’s just an issue to consider. Real problem is that you cannot install only half of maximum memory if you don’t install second CPU on 2-way server.
- CPU price, and price per core.
- Software licenses cost.

Let’s assume we chose Intel R1304WT2GS as basic platform for virtualization. So now we should to decide number of CPU to achieve maximum performance / cost efficiency. We are planning to install vSphere Standard ($1940), Windows 2012R2 Datacenter for VMs ($6155 / 2 CPUs) and Veeam Backup Standard ($1116).
Hardware costs $7970 for 1 socket configuration (E5-2670v3 12c + 192 GB), and we need licenses for 1 socket, which would be another $6133 resulting in approx. $14100 total.
2 socket configuration with the very same performance characteristics would cost $6890 (dual E5-2620v3 6с + 192 GB), but software costs double and result is $19160. So with the very same performance characteristics 1-way server is cheaper and a little bit faster due to just 1 NUMA node. Disclaimer – this is not 100% correct calculation, but a mere example of way to calculate.

So why do we need 2-way or even 4-way systems?
- 2 way servers have twice as much maximum memory.
- Single CPU is not enough for really highly loaded applications.
- As a general rule the more cores CPU have – the less is frequency. For an application with core-based licensing such as Oracle DB that can be an issue. For example, 2*E5-2637 v3 (4с, 3.50 GHz) and 1*E5-2667 v3 (8с, 3.20 GHz) have 10% difference in peak performance (28GHz vs 25,6GHz) while core-based licenses would remain the same.

General recommendations:
- Multi-core CPUs are well suited for loads with a lot of lightly to medium loaded VMs. More cores are better.
- The wider VMs are used – the better they perform.
- If you have only a few VMs with applications known for their love for MHz instead of good parallel procession – stick to higher frequency CPUs with less cores.
- Always start you design with single CPU servers. Add second CPU only after a minimum of 5 hosts reached (except of vSphere Essentials) or when you really need more memory that single CPU can support. Or leave second socket for a future upgrade.

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