On the online fault-tolerant server consolidation problem
Khuzaima Daudjee, Shahin Kamali, Alejandro López-Ortíz · 2014
In the server consolidation problem, the goal is to minimize the number of servers needed to host a set of clients. The clients appear in an online manner and each of them has a certain load. The servers have uniform capacity and the total load of clients assigned to a server must not exceed this capacity. Additionally, to have a fault-tolerant solution, the load of each client should be distributed between at least two different servers so that failure of one server avoids service interruption by migrating the load to the other servers hosting the respective second loads. In a simple setting, upon receiving a client, an online algorithm needs to select two servers and assign half of the load of the client to each server. We analyze the problem in the framework of competitive analysis. First, we provide upper and lower bounds for the competitive ratio of two well known heuristics which are introduced in the context of tenant placement in the cloud. In particular, we show their competitive ratios are no better than 2. We then present a new algorithm called Horizontal Harmonic and show that it has an improved competitive ratio which converges to 1.59. The simplicity of this algorithm makes it a good choice for use by cloud service providers. Finally, we prove a general lower bound that shows any online algorithm for the online fault-tolerant server consolidation problem has a competitive ratio of at least 1.42.