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I plan to deploy a lab environment to start testing LVS as a load
balancer in front of a group of what could be called nameservers. These
nameservers are actually serving telephone call routing, filtering, and
translation data using UDP dns-style queries, and in our production
environment normally serve 500-2000 queries per second, each.
The "clients" initiating these queries are ACME session border
controllers and various other VOIP/SIP processing equipment. A failure
of the involved systems to pass a lookup to the servers, process the
lookup, return a response, and route it back to the client is considered
critical as it means a call gets dropped or is left with "dead air".
Best case, the call gets delayed by a few seconds as a request times out
and (hopefully) gets processed by a device that is able to respond to
the retransmitted query.
I'm aware of the benefit to lowering the UDP session timeout to 15
seconds for high-volume DNS load balancing and plan to do this, but I
was wondering if LVS/IPVS incorporates methods to guarantee delivery of
a UDP request packet to a server that's able to respond to it, no matter
what.
In other words, if a DNS request comes into the VIP on the load
balancer, the load balancer forwards it (either via routing or nat) to a
"real server", but that real server is unable to correctly receive that
packet or process the query it contains for any reason, be it a dropped
packet on the wire, intermittent CPU saturation, a missed interrupt,
etc, then it would be desirable for the load balancer to detect that a
response has not been sent back to the client from the realserver and
basically re-send the same packet (same payload) to another real server
in the cluster. The typical time it takes one of these servers to
respond is usually less than 50ms, but may be as high as 100ms. If
200ms has passed after a request and the chosen server hasn't responded
yet, retransmit a copy of the original request packet to a new server
without the requesting client realizing there was a timeout.
Is this possible?
When there are 10,000 requests being processed per second, dropping even
one packet per 100,000 is disastrous for our stats.
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