Linux on Carrier Grade Web Servers
The goal of the benchmarking tests was to test the scalability of the LVS-NAT implementation. For this purpose, we carried two kinds of tests: the first one was a direct approach that consisted of sending traffic directly to the CPUs; the second approach was to direct the traffic to the NAT director that is the front-end server for the CPUs.
For the tests conducted without LVS, we sent HTTP requests directly to the real servers. WebBench supports this configuration by generating web traffic and sending them to multiple servers (Figure 5). As for the tests with LVS (Figure 6), we configured WebBench to send the HTTP requests to the LVS server (the NAT director), which in turn directed the traffic to the real servers.
Figure 7 shows the number of requests per second our LVS setup was able to achieve versus a direct setup without LVS. It clearly shows that the LVS-NAT implementation suffers from a bottleneck at the director level once it reaches 2,000 requests per second.
We decided to conduct a third test using only one machine to generate traffic with WebBench. We measured a latency of 0.3 milliseconds for answering HTTP requests. LVS handled the load successfully answering more than 178 requests per second.
After analyzing the results, we concluded that the bottleneck problem is due to the number of simultaneous TCP connections per second that the LVS director can handle. The results show that the maximum number of connections handled simultaneously by LVS is not more than 1,790 valid TCP connections per second. Without using LVS, by sending requests directly to servers, we have been able to achieve more than 7,116 valid TCP connections per second. We plan to investigate this issue in more detail in the coming weeks.
The NAT implementation of LVS has several advantages. First, the real servers can run any operating system that supports the TCP/IP protocol and they can use private internet addresses. As a result, the whole setup would only require one IP address for the load balancer.
However, the drawback of using the NAT implementation is the scalability of the virtual server. As we have seen in the benchmarking tests, the load balancer presents a bottleneck for the whole system.
LVS via NAT can meet the performance request of many small to mid-size servers. When the load balancer becomes a bottleneck, then you need to consider the other two methods offered by LVS: IP tunneling or direct routing.
We tested LVS in an industrial environment with one LVS Server and eight traffic CPUs. We found some restrictions when using LVS under heavy load. However, we also found LVS to be easy to install and manage and very useful.
We believe that LVS is a potential solution for small to mid-size web farms that need a software-based solution for traffic distribution. However, for the kind of servers we are building, the requirements necessitate a higher number of transactions per second than the NAT implementation of LVS can handle.
LVS's future is promising with the determination to add more load-balancing algorithms and geographic-based scheduling for the virtual server via IP tunneling. Another promising future feature is the integration of the heartbeat code and the CODA distributed fault-tolerant filesystem into the virtual server. LVS's developers are also planning to explore higher degrees of fault-tolerance and how to implement the virtual server in IPv6.
Compared to other packages, LVS provides many unique features such as the support for multiple-scheduling algorithms and for various methods of requests forwarding (NAT, direct routing, tunneling). Our next step regarding LVS is to try out the other two implementations (direct routing and IP tunneling) and compare the performance with the NAT implementation on the same setup.
The Systems Research Department at Ericsson Research Canada for approving the publication of this article.
Evangeline Paquin, Ericsson Research Canada, for her contributions to the overall LVS-related activities.
Marc Chatel, Ericsson Research Canada, for his considerable help in the ECUR Lab.
Wensong Zhang, the LVS Project, for the permission to use Figures 1 and 2 from the LVS web site.
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