The "uptime" is how long the server has been up since haproxy last pulled it out of rotation.
So if you have a few checks in a row that are above a certain response time, the uptime is reset.
These checks are define by the check options when defining a server:
check
This option enables health checks on
the server. By default, a server is
always considered available. If
"check" is set, the server will
receive periodic health checks to
ensure that it is really able to serve
requests. The default address and port
to send the tests to are those of the
server, and the default source is the
same as the one defined in the
backend. It is possible to change the
address using the "addr" parameter,
the port using the "port" parameter,
the source address using the "source"
address, and the interval and timers
using the "inter", "rise" and "fall"
parameters.
So the "few checks" are controlled by:
fall <count>
The "fall" parameter states that a server will be considered as dead after <count> consecutive unsuccessful health checks. This value defaults to 3 if unspecified. See also the "check", "inter" and "rise" parameters.
The response time or timers are controlled by inter:
The "inter" parameter sets the
interval between two consecutive
health checks to milliseconds.
If left unspecified, the delay
defaults to 2000 ms. It is also
possible to use "fastinter" and
"downinter" to optimize delays between
checks depending on the server state :
Server state Interval used Up 100%
(non-transitional) "inter"
Transitionally up (going down)
Transitionally down (going up), or yet
unchecked "fastinter" if set, "inter"
otherwise Down 100%
(non-transitional) "downinter" if
set, "inter" otherwise Just as with
every other time-based parameter, they
can be entered in any other explicit
unit among { us, ms, s, m, h, d }. The
"inter" parameter also serves as a
timeout for health checks sent to
servers if timeout check is not set.
In order to reduce "resonance" effects
when multiple servers are hosted on
the same hardware, the health-checks
of all servers are started with a
small time offset between them. It is
also possible to add some random noise
in the health checks interval using
the global "spread-checks" keyword.
This makes sense for instance when a
lot of backends use the same servers.
More at http://code.google.com/p/haproxy-docs/wiki/ServerOptions .
