Red Hat ENTERPRISE LINUX 4 Manual

Also see for ENTERPRISE LINUX 4 - DEVELOPER TOOLS GUIDE: Installation guide Installation guide Tuning guide Installation guide Manual

Contents

Chapter 1. Red Hat Cluster Suite Overview28Service requests arriving at an LVS router are addressed to a virtual IP address or VIP. This is apublicly-routable address that the administrator of the site associates with a fully-qualified domainname, such as www.example.com, and which is assigned to one or more virtual servers1. Note that aVIP address migrates from one LVS router to the other during a failover, thus maintaining a presenceat that IP address, also known as floating IP addresses.VIP addresses may be aliased to the same device that connects the LVS router to the public network.For instance, if eth0 is connected to the Internet, then multiple virtual servers can be aliased toeth0:1. Alternatively, each virtual server can be associated with a separate device per service. Forexample, HTTP traffic can be handled on eth0:1, and FTP traffic can be handled on eth0:2.Only one LVS router is active at a time. The role of the active LVS router is to redirect service requestsfrom virtual IP addresses to the real servers. The redirection is based on one of eight load-balancingalgorithms:• Round-Robin Scheduling — Distributes each request sequentially around a pool of real servers.Using this algorithm, all the real servers are treated as equals without regard to capacity or load.• Weighted Round-Robin Scheduling — Distributes each request sequentially around a pool ofreal servers but gives more jobs to servers with greater capacity. Capacity is indicated by a user-assigned weight factor, which is then adjusted up or down by dynamic load information. This is apreferred choice if there are significant differences in the capacity of real servers in a server pool.However, if the request load varies dramatically, a more heavily weighted server may answer morethan its share of requests.• Least-Connection — Distributes more requests to real servers with fewer active connections. Thisis a type of dynamic scheduling algorithm, making it a better choice if there is a high degree ofvariation in the request load. It is best suited for a real server pool where each server node hasroughly the same capacity. If the real servers have varying capabilities, weighted least-connectionscheduling is a better choice.• Weighted Least-Connections (default) — Distributes more requests to servers with fewer activeconnections relative to their capacities. Capacity is indicated by a user-assigned weight, whichis then adjusted up or down by dynamic load information. The addition of weighting makes thisalgorithm ideal when the real server pool contains hardware of varying capacity.• Locality-Based Least-Connection Scheduling — Distributes more requests to servers with feweractive connections relative to their destination IPs. This algorithm is for use in a proxy-cache servercluster. It routes the packets for an IP address to the server for that address unless that server isabove its capacity and has a server in its half load, in which case it assigns the IP address to theleast loaded real server.• Locality-Based Least-Connection Scheduling with Replication Scheduling — Distributes morerequests to servers with fewer active connections relative to their destination IPs. This algorithmis also for use in a proxy-cache server cluster. It differs from Locality-Based Least-ConnectionScheduling by mapping the target IP address to a subset of real server nodes. Requests are thenrouted to the server in this subset with the lowest number of connections. If all the nodes for thedestination IP are above capacity, it replicates a new server for that destination IP address byadding the real server with the least connections from the overall pool of real servers to the subsetof real servers for that destination IP. The most-loaded node is then dropped from the real serversubset to prevent over-replication.1 A virtual server is a service configured to listen on a specific virtual IP.