
Load Balancing and High Availability
|
157
The installation process will ask you some questions:
Do you want to automatically load IPVS rules on boot?
No
Select a daemon method.
none
Our configuration will have one virtual server (the address that clients see, running
ldirectord), which we’ll call the director, and two realservers (running Apache). The
realservers can be connected to the director in one of three ways:
LVS-NAT
The realservers are in a NAT subnet behind the director and route their
responses back through the director.
LVS-DR
The realservers route their responses directly back to the client. All machines are
on the same subnet and can find each other’s level-2 (Ethernet) addresses. They
do not need to be pingable from outside their subnet.
LVS-TUN
The realservers can be on a different network from the director. They communi-
cate by tunneling with IP-over-IP (IPIP) encapsulation.
We’re going to use DR, because it’s easy, it’s fast, and it scales well. With this
method, we designate a VIP that is shared by the load balancer and the realservers.
This causes an immediate problem: if all machines share the same VIP, how do we
resolve the VIP to a single physical MAC address? This is called the ARP problem,
because systems on the same LAN use the Address Resolution Protocol (ARP) to find
each other, and ARP expects each system to have a unique IP address.
Many solutions require kernel patches or ...