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.
