Linux load balancer software one of the LVS (concept article)

First, the LVS introduction LVS is the abbreviation of Linux virtual server, that is, the Linux virtualized servers, is a free software project initiated by Dr. Zhangwensong, its official site is www.linuxvirtualserver.org. Now LVS is already part of the Linux standard kernel, Prior to the Linux2.4 kernel, the LVS had to be recompiled to support the LVS function module, but since the Linux2.4 kernel, the various functions of LVS have been built-in, without any patching of the kernel, and the functions provided by LVS can be used directly. The goal of using LVS technology is to achieve a high-performance, highly available server cluster with the load balancing technology provided by LVS and the Linux operating system, which has good reliability, scalability and operability. To achieve the best service performance at low cost. LVS since 1998, has developed into a more mature technology project now. LVS technology can be used to achieve highly scalable, highly available network services, such as WWW services, cache services, DNS services, FTP services, mail services, video/audio-on-demand services, and so on, there are many more famous sites and organizations are using LVS set up the cluster system, For example: The Linux portal (www.linux.com), real Company (www.real.com), the world's largest open source website (sourceforge.net), which provides audio and video services to RealPlayer. The LVS architecture uses the LVS to set up the server cluster system has three parts: the most front-end load balancing layer, with load balancer, the middle server group layer, with server array, the bottom of the data sharing storage layer, with shared Storage said that in the eyes of users, all internal applications are transparent, users are only using a virtual server to provide high-performance services. The LVS architecture is shown in 1:

Figure 1 Architecture of LVS

Below is a detailed description of the various components of LVS:? Load Balancer layer: At the forefront of the entire cluster system, there is one or more load scheduler (Director server), the LVS module is installed on the director server, and director's main role is similar to a router, It contains the routing tables set up to complete the LVS function, which distribute the user's requests to the application server (Real server) at the server array level through these routing tables. Also, on the director server, you install the Monitoring module Ldirectord for the real Server service, which is used to monitor the health status of each real Server service. When real server is unavailable, remove it from the LVS routing table and rejoin it upon recovery. Server Array layer: Consists of a set of machines that actually run the app service, one or more of the Web server, mail server, FTP server, DNS server, video server, and each real Servers are connected to each other over a high-speed LAN or across a WAN. In real-world applications, the Director server can also be the role of real server concurrently. Shared storage layer: is a storage area that provides shared storage space and content consistency for all real servers, physically consisting of disk array devices and, in order to provide consistency of content, can generally share data via NFS Network file systems. But NFS in a busy business system, performance is not very good, at this time can use the cluster file system, such as Red Hat GFs file system, Oracle provides the OCFS2 file system and so on. As can be seen from the entire LVS structure, director server is the core of the entire LVS, currently, the operating system for director server can only be Linux and FreeBSD, The linux2.6 kernel can support LVS without any setup, and FreeBSD as a director server is not a lot of applications, performance is not very good. For real Server, almost all system platforms, Linux, Windows, Solaris, AIX, BSD series can be very well supported.

Three, the characteristics of LVS cluster 3.1 IP load balancing and load scheduling algorithm

1. There are many implementations of IP load balancing technology, such as the method of DNS domain name rotation resolution, the method based on client scheduling access, the scheduling method based on application layer system load, and the scheduling method based on IP address, in which the highest execution efficiency is the IP load balancing technology. The IP load balancing technology of LVS is realized by Ipvs module, Ipvs is the core software of LVS cluster system, its main function is: Install on Director server, and virtual an IP address on Director server. The user must access the service through this virtual IP address. This virtual IP is generally called the LVS VIP, namely virtual IP. The requests that are accessed first go through the VIP to the load scheduler, and then the load Scheduler picks a service node from the real server list to respond to the user's request. When the user's request arrives at the load scheduler, how the scheduler sends the request to the real server node that provides the service, and how the real server node returns the data to the user, is the key technology implemented by Ipvs, and there are three kinds of load balancing mechanisms Ipvs, namely NAT, Tun, and Dr, The details are as follows:  vs/nat: That is (virtual server via network address translation), which is the translation technology of Web addresses to implement a virtualized server, when the user requests to reach the scheduler, The scheduler overwrites the destination address of the request message (that is, the virtual IP address) to the selected real server address, while the destination port of the message is also changed to the corresponding port of the selected real server, and finally the message request is sent to the selected real server. After the data is obtained on the server side, when Real server returns the data to the user, it needs to go through the load scheduler again to change the source address and source port of the message to the virtual IP address and the corresponding port, then send the data to the user to complete the load scheduling process. It can be seen that in the NAT mode, the user request and response messages must be rewritten by the Director Server address, when the user requests more and more time, the scheduler's processing power will be called bottlenecks.  vs/tun: That is (Virtual Server via IP Tunneling) is also the IP tunneling technology to implement virtual server. Its connection scheduling and management is the same as the Vs/nat way, but its message forwarding method is different, Vs/tun mode, the Scheduler uses IP tunneling technology to forward user requests to a real server, and this real server will directly respond to the user's request, no longer through the front-end scheduler, In addition, the geographic location of the real serverThere is no requirement to be in the same network segment as the director server or as a standalone network. Therefore, in the Tun mode, the scheduler will only process the user's message request, the throughput of the cluster system is greatly improved.  VS/DR: That is, virtual server via direct Routing, that is, the use of direct routing technology to implement the VM. Its connection scheduling and management is the same as in Vs/nat and Vs/tun, but its message forwarding method is different, vs/dr by overwriting the request message's MAC address, send the request to real server, and real server to return the response directly to the customer, eliminating the vs/ The IP tunneling overhead in the Tun. This is the best performance in three load scheduling mechanisms, but it must be required that both the Director server and the real server have a NIC attached to the same physical network segment.

2. Load scheduling algorithm above we talked about, load scheduler is based on the load situation of each server, dynamically select a real server to respond to user requests, then the dynamic selection is how to implement, in fact, we are here to say the load scheduling algorithm, according to different network service requirements and server configuration, Ipvs implements the following eight kinds of load scheduling algorithms, here we detail the most commonly used four scheduling algorithms, the remaining four scheduling algorithms please refer to other information.?  Round call Dispatch (Round Robin) "Round call" dispatch also called 1:1 Dispatch, scheduler through "round call" The scheduling algorithm assigns external user requests to each real server in the cluster in order 1:1, which treats each real server equally, regardless of the actual load condition and connection state on the server.?  weighted round call scheduling (Weighted Round Robin) The "Weighted round call" scheduling algorithm dispatches access requests based on the different processing capabilities of real server. You can set different scheduling weights for each real server, and for a relatively good real server, you can set a higher weight, and for a less powerful real server, you can set a lower weight value, which ensures that the processing power of the server handles more traffic. The server resources are utilized fully and rationally. At the same time, the scheduler can automatically query the load situation of real server and dynamically adjust its weights.?  Minimum link scheduling (Least Connections) The "least connection" scheduling algorithm dynamically dispatches network requests to servers with the fewest number of established links. If the real server of the cluster system has similar system performance, the "Minimum connection" scheduling algorithm can well balance the load.?  weighted least-link scheduling (Weighted Least Connections) "Weighted least-link scheduling" is a superset of "least connection scheduling", Each service node can represent its processing power with corresponding weights, while the system administrator can dynamically set the corresponding weights, the default weight is 1, and the weighted minimum connection schedule is proportional to the number of established connections of the service node and its weights as much as possible when allocating new connection requests. The other four scheduling algorithms are: local-based least-link (locality-based Least Connections), local-based least-link with replication (locality-based Least Connections with Replication), the destination address hash (Destination Hashing), and the source address hash (sources Hashing), for the meaning of these four scheduling algorithms, this article no longer speaks, if you want to know more about the remaining four scheduling strategies, you can log in to the LVS Chinese site zh.linuxvirtualserver.org for more detailed information.

3.2 High Availability LVS is a kernel-level application software, so it has high processing performance, and the load-balanced cluster system with LVS architecture has excellent processing ability, the fault of each service node will not affect the normal use of the whole system, at the same time realize the reasonable balance of load, Enables the application to have an extremely high-load service capability that supports millions of concurrent connection requests. If you configure a Gigabit network card, using Vs/tun or VS/DR scheduling technology, the entire cluster system throughput can be as high as 1gbits/s, such as the configuration of gigabit network cards, the maximum throughput of the system is close to 10gbits/s.

3.3 High reliability LVS Load Balancer Cluster software has been widely used in enterprises, schools and other industries, many large, critical Web sites are also using the LVS cluster software, so its reliability in practice has been very well confirmed. There are a lot of LVS-made load-balancing systems that run for a long time and have never been restarted. These demonstrate the high stability and high reliability of LVS.

3.4 Applicable environment LVs to front-end director server currently only supports Linux and FreeBSD systems, but supports most TCP and UDP protocols, applications that support TCP protocols are: Http,https, FTP,SMTP,,POP3,IMAP4, Proxy,ldap,ssmtp and so on. The applications that support UDP protocol are: DNS,NTP,ICP, video, audio streaming protocol and so on. LVS has no limitations on real server's operating system, and real server can run on any TCP/IP-enabled operating system, including Linux, Unix (such as FreeBSD, Sun Solaris, HP UNIX, etc.), mac/ OS, Windows, and more.

3.5 Open Source Software The LVS cluster software is a free software issued under the GPL (GNU public License) license, so the user can obtain the source code of the software, and can make various changes according to their own needs, but the modification must be distributed in GPL mode.

Linux load balancer software one of the LVS (concept article)