IPVS
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.
Contents
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 Transaltion
- 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.
Design and Implementation
Ongoing Development
IPVS On FreeBSD
The LVS On FreeBSD project was started by Li Wang, the goal is to port IPVS code to FreeBSD. Now, the LVS On FreeBSD module supports the LVS/DR and LVS/TUN ip load balancing technologies.
