Service checks when host is down

[Andreas Ericsson]

Az wrote:
> Hari Sekhon wrote:
>> Indeed, that would be ok too, since then you can disable the host 
>> check and rely entirely on service checks so you get all the service 
>> checks.
>>
>> Almost as good. Of course it won't say host down, but it will say ping 
>> failed so you can tell from that.
>>
>> Another down side is that it will pollute your web interface with 
>> loads of trivial services, whereas it nice at the moment when all you 
>> see is the intended service.
>>
>> It would kind of spoil the web interface in my opinion.
>>
>> I think I will just wait for this to be done at the developer side, so 
>> that it is part of the normal configuration options.
> You may also find that it will blow out your service check execution 
> times if you use a ping (service) check of, say, 5 packets @ 1 second 
> apart. It doesnt sound like much but if you have a large number of hosts 
> it *might* cause you problems.
> 

I can most definitely say that it won't. A typical ICMP packet is 64 
bytes, all headers included. If you're thinking about load issues, 99% 
of the time check_icmp, /bin/ping and other ping-like programs use are 
spent waiting for data from the host being pinged. 0.8% of the time is 
spent by the system setting up the stack and creating memory pages for 
the program, and 0.2% is spent sending packets, calculating 
response-times and things like that. I can imagine that this will cause 
problems if you're monitoring tens of thousands of hosts, but if you do 
that without a robust clustering solution or some insanely beefy 
hardware and network line you'll be toast anyways.

check_http, or any other plugin that generally fetches >1KiB of data, 
possibly decrypts it, and parses a bit of text to be able to return a 
result are easily a hundred times heavier on both systemload and network 
traffic than ping checks.

-- 
Andreas Ericsson                   andreas.ericsson at op5.se
OP5 AB                             www.op5.se
Tel: +46 8-230225                  Fax: +46 8-230231

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