1. ElasticTree: Saving Energy in Data Center Networks Very offended by KALYAN MANDA LEI XIA

2. Facts about Network Elements Network Elements only consume 5% of Total Power and not 10-20% as stated in your paper.(hmmm……..) ElasticTree can save upto 40% of network energy, i.e. we have saved 1billion kWh.(best case scenario) This is achieved by deciding on trade-off between Performance and Energy Consumption.

3. Some Maths to calculate the Profit Ok, so we have just saved: = 1 *10^9 *.088(price/kWh) = $88000000. Number of servers in Datacenters in 2007: = 11.8Million. (http://royal.pingdom.com/2008/07/25/us-data-centers-consuming-as-much-power-as-5-million- houses/)http://royal.pingdom.com/2008/07/25/us-data-centers-consuming-as-much-power-as-5-million- houses/ Approx. number of Datacenters: = 11.8*10^6/10000(a very generous assumption) = 1180

4. Profit/Datacenter = 88000000 / 1180 = $74576.27/year HURRAY!!!

5. Wait a Second…. I will have to hire a Network Admin to configure ElasticTree to satisfy the needs for performance and Fault tolerance. (source - paper) And since my datacenter will be running 24/7 will have to hire 3 network admins- one per shift.(ideally 6- other 3 as backup) And since I am a miser I will be paying them only $60000 pa. So my total investment = 3 * 60000 = $180000/ year So my Total Profit = 74576.27 – 180000 = $-105423.73

6. My Achievements I have degraded the performance of my datacenter by selecting on a trade-off between energy and performance to save 40% network energy…assuming the best case scenario. I have made a profit of $-105423.73 And, I got myself fired…..

7. Applications of ElasticTree Only to datacenters with Fat tree topology at present. Cannot be applied to a 2N Tree topology. Future interest, to explore the applicabilty to other topologies such as HyperCubes and Butterfly topology.

8. Common guys…… You based all of your conclusions by performing tests on a k = 4,6 Fat Tree and with a maximum of 54 hosts. How hard it could be to simulate other topologies, considering the fact that you have not simulated on a real datacenter. This also clearly indicates the lack of interest of Stanford university, Duetsche Telekom and HP labs – your employers, in your work.

9. ElasticTree v/s Third Party Computing Will Elastic Tree approach be more profitable than Third Party Computing? Wouldn’t it make more sense to outsource my datacenter’s additional capacity to third party rather than shutting them so as to save power costs? Considering the fact that VM’s are instantiated on demand, will the optimizer algorithm be any good since it needs a lot of previous information?

10. ElasticTree v/s DDOS Wouldn’t a datacenter running on a ElasticTree be an ideal target for a DDOS attack. Since there is no way to predict a DDOS attack even from previous statistics???

11. Robustness Analysis A datacenter will use a MST-k configuration to absorb k simultaneous faults. With a MST-k configuration our total energy savings = 5% * 40% * Total Power k

12. Testbed: Far from the real Switches: Virtual Switch? ▫Power consumption, on/off ▫Latency ▫Configuration operability Topology: simplified fat tree (k=4) ▫Real DCs: k>12~ ▫Other topologies, not only fat tree

13. Testbed: Far from the real – cont’d Workload Traffic: ▫collected from DC with only 290 servers ▫different fat tree configurations with simulation. ▫Traffic collected in very coarse granularity.  10 minutes interval  Anything could happen during this interval in real DC (burst, raise/down)

14. System response time The algorithms itself ▫3.5 th order for formal model ▫2th order for greedy ▫How many time will these take in real system? ▫What the response time? ▫None of these are given in the paper.

15. Real Switch Latency Time to boot a real switch ▫Up to 3 mins? ▫How to handle this network traffic burst  The latency time is unbearable in most of DCs ▫Authors propose us to use the possible fantasy feathers from these future switches, like sleeping mode

16. How Control Module works in real DC? How the control signals sent out ▫In simulation, easy, all software ▫In real DC, how?  By extra wires from control machine to each switch?  You justify that in real DC one can use mechanisms such as command line interface, SNMP etc to on/off switches, why not use them rather than state them…… OpenFlow & NOX ▫Does widely used switches and DC has these? ▫How many efforts to port these?

17. Centralized Control Module? – not good idea Control modules – single point of failure ▫What is the potential harmfulness of control failure?  If down, the whole DC could be malfunctioned, or stopped ▫What is the recovery process from failure  Cost/time

18. Results – Not convinced What the overall power saving for the real DC traffics? In figures 15 and 16. What is the typical latency and lost percentage for DCs without ElasticTree applied? ▫Authors claim the results from figures 15 and 16 are helpful for DC operators to choose needed tradeoff,  Actually not!!  The statistics differ a lot for DCs with different configurations and scales