"Load Balancing": How Busy are the servers?

[Chuck Swiger]

Marc G. Fournier wrote:
> 1. What variables on a server should be monitored to determine how busy 
> a server is?  For instance, I've always been taugth that 'loadavg' is 
> not an indication of how busy a server is, since a high loadavg on a 
> single CPU server might be an overloaded server, but moderately loaded 
> on a dual CPU server ...

If the load average is greater than the number of CPU's, it's likely that adding 
another CPU would improve throughput.  If the load average is more than twice 
the # of CPU's it's extremely likeing that adding more CPUs would improve 
throughput.

> disk i/o, cpu usage, ethernet throughput ... what else?

The primary resources are CPU, memory, and I/O.  If you measure the ones you've 
listed and pay attention to the VM stats, you should have a starting point. 
Don't forget to pay attention to running out of disk space, SysV shmem 
semaphores, and anything else which is being used by the tasks being run.

> 2. Are there any tools that I can run to give me a point in time 
> "summary" of how busy a server is based on these several factors?

"vmstat 5" and "iostat 5" come pretty close, but you have to calibrate some of 
the I/O measurements it returns against the maximum throughput possible for each 
specific system.

> Basically, I'd like to keep track of multiple servers and be able to say 
> "this server is running >75% of capacity, time to upgrade or move things 
> off of it" ... if its possible ... ?

Take a look at Big Brother, www.bb4.org?, it will at least give warnings for 
high load average, disk space, and so forth.

-- 
-Chuck