Investigating Slow Linux Server

Question

I know this is such a familiar old question, but I'm a bit stumped.

I have a Rackspace virtual server running Fedora 14. It's a small machine, doing not a great deal (a test web site) with 256MB of RAM and 10GB of disk.

In the last few days it's started operating really slowly. It's slow in a terminal session, programs are taking a long time to run and the website is bogging now or becomes inoperable.

I've checked what I think are the obvious things - top isn't showing any heavy CPU load, there's free memory available and my Nagios monitoring isn't showing any load either.

I created the machine with Puppet and last night I created an 'identical' (I presume) machine that is running fine.

I've checked TCP to see if there's anything going in and out that might be a bottleneck and that's not revealed anything either.

Are there any other tips anyone can offer? I don't understand why it should be slow yet I can find no measurable evidence of this slowness other than what I experience.

Answer 1

The thing to do is to open a ticket with Rackspace and have them sort it out, that's why you are paying them after all.

My money is on this being a disk i/o issue with them having either heavily oversubscribed their low end VPS offering or a different VPS user gobbling up all of the disk i/o.

In either case though, in VPS deployments you often have to involve the host because you as VPS user do not have any insight on what is happening on the physical hardware. It's back to opening a ticket with the provider.

Answer 2

Some general tips in no particular order:

• Get atop installed on there. It logs system activity since boot, a bit like sar, but gives a nice interface to analyse it. Look for high usage processes in memory, CPU, disk and network.
• Look for high %wa on CPU, this is the CPU waiting on external I/O, normally caused by slow disk access and especially in a VPS environment. I would be hedging my bets here as it's such a common cause of problems. If you are seeing high %wa then there's nothing you will be able to do about it as it's normally caused by overstretched storage. Shout at the ISP until they fix it!
• 256M of memory isn't a lot, especially if the website in question is built on a framework (eg Symfony, django, etc) and running something like Apache. That combination can gobble memory up if not properly configured. See if your system is using any swap, and if it is then you need either more memory or you need to limit the memory usage of your web server via configuration. If you're running Apache, look at the options under the prefork settings.

This does sound like overstretched host hardware unfortunately, and the evidence for that will be apparent from CPU wait time. That figure in a VPS environment can be summed up as "any issue outside of the guest virtual machine"!

Ask them to migrate you to a different host if possible as a benchmark test.