|Red Hat Cluster Suite: Configuring and Managing a Cluster|
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In certain situations, it may be desirable for a client to reconnect repeatedly to the same real server, rather than have an LVS load balancing algorithm send that request to the best available server. Examples of such situations include multi-screen web forms, cookies, SSL, and FTP connections. In these cases, a client may not work properly unless the transactions are being handled by the same server to retain context. LVS provides two different features to handle this: persistence and firewall marks.
When enabled, persistence acts like a timer. When a client connects to a service, LVS remembers the last connection for a specified period of time. If that same client IP address connects again within that period, it is sent to the same server it connected to previously — bypassing the load-balancing mechanisms. When a connection occurs outside the time window, it is handled according to the scheduling rules in place.
Persistence also allows the administrator to specify a subnet mask to apply to the client IP address test as a tool for controlling what addresses have a higher level of persistence, thereby grouping connections to that subnet.
Grouping connections destined for different ports can be important for protocols which use more than one port to communicate, such as FTP. However, persistence is not the most efficient way to deal with the problem of grouping together connections destined for different ports. For these situations, it is best to use firewall marks.
Firewall marks are an easy and efficient way to a group ports used for a protocol or group of related protocols. For instance, if an LVS cluster is deployed to run an e-commerce site, firewall marks can be used to bundle HTTP connections on port 80 and secure, HTTPS connections on port 443. By assigning the same firewall mark to the virtual server for each protocol, state information for the transaction can be preserved because the LVS router forwards all requests to the same real server after a connection is opened.
Because of its efficiency and ease-of-use, administrators of LVS clusters should use firewall marks instead of persistence whenever possible for grouping connections. However, administrators should still add persistence to the virtual servers in conjunction with firewall marks to ensure the clients are reconnected to the same server for an adequate period of time.