1.3. LVS Scheduling Overview

1.3. LVS Scheduling Overview

One of the advantages of using an LVS cluster is its ability to perform flexible, IP-level load balancing on the real server pool. This flexibility is due to the variety of scheduling algorithms an administrator can choose from when configuring a cluster. LVS load balancing is superior to less flexible methods, such as Round-Robin DNS where the hierarchical nature of DNS and the caching by client machines can lead to load imbalances. Additionally, the low-level filtering employed by the LVS router has advantages over application-level request forwarding because balancing loads at the network packet level causes minimal computational overhead and allows for greater scalability.

Using scheduling, the active router can take into account the real servers' activity and, optionally, an administrator-assigned weight factor when routing service requests. Using assigned weights gives arbitrary priorities to individual machines. Using this form of scheduling, it is possible to create a group of real servers using a variety of hardware and software combinations and the active router can evenly load each real server.

The scheduling mechanism for an LVS cluster is provided by a collection of kernel patches called IP Virtual Server or IPVS modules. These modules enable layer 4 (L4) transport layer switching, which is designed to work well with multiple servers on a single IP address.

To track and route packets to the real servers efficiently, IPVS builds an IPVS table in the kernel. This table is used by the active LVS router to redirect requests from a virtual server address to and returning from real servers in the pool. The IPVS table is constantly updated by a utility called ipvsadm — adding and removing cluster members depending on their availability.

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