Linux Virtual Server
Linux Virtual Server (LVS) is load balancing software for Linux systems. It is an open source project started by Wensong Zhang in May 1998. The mission of the project is to build a high-performance and highly available server for Linux using clustering technology, which provides good scalability, reliability and serviceability.
Purpose and function 
The major work of the LVS project is now to develop advanced IP load balancing software (IPVS), application-level load balancing software (KTCPVS), and cluster management components.
- IPVS: is an advanced IP load balancing software implemented inside the Linux kernel. The IP Virtual Server code was already included into the standard Linux kernel 2.4 and 2.6.
- KTCPVS: implements application-level load balancing inside the Linux kernel, currently under development.
Users can use the LVS solutions to build highly scalable and highly available network services, such as web, email, media services and VoIP services, and integrate scalable network services into large-scale reliable e-commerce or e-government applications.
The LVS solutions have already been deployed in many real applications throughout the world, including Wikipedia.
The LVS component depends upon the Linux Netfilter framework and its source code is available in the net/netfilter/ipvs/ subdirectory within the kernel source. It implements several balancing schedulers, listed below with the relevant source file:
- Round-Robin (ip_vs_rr.c)
- Weighted Round-Robin (ip_vs_wrr.c)
- Least-Connection (ip_vs_lc.c)
- Weighted Least-Connection (ip_vs_wlc.c)
- Locality-Based Least-Connection (ip_vs_lblc.c)
- Locality-Based Least-Connection with Replication (ip_vs_lblcr.c)
- Destination Hashing (ip_vs_dh.c)
- Source Hashing (ip_vs_sh.c)
- Shortest Expected Delay (ip_vs_sed.c)
- Never Queue (ip_vs_nq.c)
List retrieved from http://www.linuxvirtualserver.org/docs/scheduling.html
The module is able to handle UDP, TCP layer-4 protocols as well as FTP passive connection by inspecting layer-7 packets. It provides a hierarchy of counters in the /proc/ directory.
The userland tool is ipvsadm.
Major terms 
- LVS DIRECTOR or simply Director: The load balancer that receives all incoming client requests for services and directs them to a specific "real server" to handle the request.
- REAL SERVERS: The nodes that make up an LVS cluster which are used to provide services on the behalf of the cluster.
- CLIENT COMPUTERS: The computers requesting services from the Virtual Server.
IP address names 
- VIP - Virtual IP address: The IP address used by the Director to provide services to client computers.
- RIP - Real IP address: The IP address used to connect to the cluster nodes.
- DIP - Directors IP address: The IP address used by the Director to connect to network of Real IP addresses.
- CIP - Client IP address: The IP address assigned to a client computer, that it uses as the source IP address for requests being sent to the cluster.