IPVS (IP Virtual Server) implements transport-layer load balancing inside the Linux kernel, so called Layer-4 switching. IPVS running on a host acts as a load balancer before a cluster of real servers, it can direct requests for TCP/UDP based services to the real servers, and makes services of the real servers to appear as a virtual service on a single IP address.
IP Load Balancing Technologies
Since the IP load balancing techniques have good scalability, IPVS extends the TCP/IP stack of the Linux kernel to support three IP load balancing techniques: LVS/NAT, LVS/TUN and LVS/DR. The box running IPVS acts as a load balancer of network connections from clients who know a single IP address for a service, and distributes them to a set of servers that actually perform the work.
- Virtual Server via Network Address Translation
- Virtual Server via IP Tunneling
- Virtual Server via Direct Routing
Job Scheduling Algorithms
Connection Scheduling Algorithms inside the Kernel
IPVS schedules jobs at connection granularity inside the Linux kernel. Here connection is defined as data communication between client socket and server socket, no matter whether it uses TCP or UDP protocol. For scheduling UDP datagrams, IPVS load balancer records UDP datagram scheduling with configurable timeout, and the default UDP timeout is 300 seconds. Before UDP connection timeouts, all UDP datagrams from the same socket (protocol, ip address and port) will be directed to the same server.
IPVS has implemented ten connection scheduling algorithms inside the kernel so far:
- Round-Robin Scheduling
- Weighted Round-Robin Scheduling
- Least-Connection Scheduling
- Weighted Least-Connection Scheduling
- Locality-Based Least-Connection Scheduling
- Locality-Based Least-Connection with Replication Scheduling
- Destination Hashing Scheduling
- Source Hashing Scheduling
- Shortest Expected Delay Scheduling
- Never Queue Scheduling
Dynamic Feedback Load Balancing Scheduling
The dynamic feedback load balancing scheduling algorithm is to use connection scheduling algorithm and adjust server weight based on dynamic feeback load information of each server, in order to avoid load imbalance among servers.
See http://www.linuxvirtualserver.org/software/ipvs.html for downloading IPVS related software.
ipvsadm is used to set up, maintain or inspect the IP virtual server table in the Linux kernel. For how to compile ipvsadm on on different Linux distributions, check this article for detailed information.
Design and Implementation
- IPVS Design
- IPVS Implementation
- IPv6 load balancing - the status of IPv6 support in IPVS and examples