How can I check why a Varnish probe fails?

Question

I'm used to run

varnishadm -T localhost:6082 debug.health

to check the backends health status, but how can I check why a probe fails in details (eg timeouts, wrong http status code)?

Answer 1

In Varnish 4.0 you can see the status of all backends and their recent probe success rate with

varnishadm backend.list

Answer 2

A little tricky to find [1] but:

Every poll is recorded in the shared memory log as follows:

NB: subject to polishing before 2.0 is released!

0 Backend_health - b0 Still healthy 4--X-S-RH 9 8 10 0.029291 0.030875 HTTP/1.1 200 Ok

...

Notice that the second word indicates present state, and the first word == "Still" indicates unchanged state.

• 4--X-S-RH -- Flags indicating how the latest poll went
  • 4 -- IPv4 connection established
  • 6 -- IPv6 connection established
  • x -- Request transmit failed
  • X -- Request transmit succeeded
  • s -- TCP socket shutdown failed
  • S -- TCP socket shutdown succeeded
  • r -- Read response failed
  • R -- Read response succeeded
  • H -- Happy with result
• 9 -- Number of good polls in the last .window polls
• 8 -- .threshold (see above)
• 10 -- .window (see above)
• 0.029291 -- Response time this poll or zero if it failed
• 0.030875 -- Exponential average (r=4) of responsetime for good polls.
• HTTP/1.1 200 Ok -- The HTTP response from the backend.

So you should use varnishlog to get fail details.

[1] https://www.varnish-cache.org/trac/wiki/BackendPolling#SHMlog