Linux Virtual Server (LVS) is load balancing software for

Linux kernel The Linux kernel is a free and open-source, monolithic, modular, multitasking, Unix-like operating system kernel. It was originally authored in 1991 by Linus Torvalds for his i386-based PC, and it was soon adopted as the kernel for the GNU ...

–based operating systems. LVS is a

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project started by Wensong Zhang in May 1998, subject to the requirements of the

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(GPL), version 2. 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.

Overview

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: an advanced IP load balancing software implemented inside the

. The

IP Virtual Server IPVS (IP Virtual Server) implements transport-layer load balancing, usually called Layer 4 LAN switching, as part of the Linux kernel. It's configured via the user-space utility tool. IPVS is incorporated into the Linux Virtual Server (LVS), ...

code is merged into versions 2.4.x and newer of the Linux kernel mainline. * KTCPVS: implements application-level load balancing inside the Linux kernel, still under development. LVS can be used for building highly scalable and highly available network services, such as web, email, media and

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services, and integrating scalable network services into large-scale reliable

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applications. LVS-based solutions already have been deployed in many real applications throughout the world, including

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. The LVS components depend upon the Linux

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framework, and its source code is available in the net/netfilter/ipvs subdirectory within the

source. LVS 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 utility program used to configure LVS is called ipvsadm, which requires

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privileges to run.

Schedulers

LVS implements several balancing schedulers, listed below with the relevant source files: * 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) * Maglev hashing (ip_vs_mh.c)

Glossary

Commonly used terms include the following: * ''LVS director'': load balancer that receives all incoming client requests for services and directs them to a specific "real server" to handle the request * ''Real servers'': nodes that make up an LVS cluster which are used to provide services on the behalf of the cluster * ''Client computers'': computers requesting services from the virtual server * ''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

Examples

Setting up a virtual

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server with two real servers: ipvsadm -A -t 192.168.0.1:80 -s rr ipvsadm -a -t 192.168.0.1:80 -r 172.16.0.1:80 -m ipvsadm -a -t 192.168.0.1:80 -r 172.16.0.2:80 -m The first command assigns TCP port 80 on

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192.168.0.1 to the virtual server. The chosen scheduling algorithm for load balancing is round-robin (-s rr). The second and third commands are adding IP addresses of real servers to the LVS setup. The forwarded network packets shall be masked (-m). Querying the status of the above configured LVS setup: # ipvsadm -L -n IP Virtual Server version 1.0.8 (size=65536) Prot LocalAddress:Port Scheduler Flags -> RemoteAddress:Port Forward Weight ActiveConn InActConn TCP 192.168.0.1:80 rr -> 172.16.0.2:80 Masq 1 3 1 -> 172.16.0.1:80 Masq 1 4 0

References

External links