We just improved Dawn, our website monitoring tool, to help prevent and identify another common cause of website outages: disk thrashing.
One common cause of disk thrashing is excessive use of memory (RAM). This may be due to a large volume of traffic hitting your server, or complex pages which require significant amounts of memory to produce. In those situations, the web server begins using the hard disk instead of RAM, with dramatic consequences to performance: A platter hard disk is considerably slower than RAM, causing the server to slow to a crawl.
Swapping some memory to hard disk is acceptable in certain situations, for example, when you have Photoshop running in the background on your desktop machine. However, when a web server such as Apache begins swapping memory to hard disk to serve web traffic, it generates too much hard disk activity, which results in a severe system-wide bottleneck. You could find yourself with a web server not serving any traffic at all while hard disk and CPU activity are complete maxed out.
It's important for a website monitoring tool such as Dawn to anticipate and measure this kind of problem. In the past, Dawn has recorded physical memory usage. However, in a normal environment, you will have very little free RAM. Any RAM not used by running software will typically be consumed by file buffers: copies of frequently accessed files held in RAM to improve performance of the machine. It's therefore not useful for Dawn to show that your physical RAM is typically mostly used. What you do want to know is when the memory devoted to running programs has exhausting your RAM and thus is in danger of disk thrashing. Therefore, Dawn only measures "used" memory, excluding buffers usage and swap space.
Lastly, Dawn also measures the rate at which information is being swapped in our out, as this is effectively the measure of thrashing that your server is doing. Measuring this lets you instantly understand whether or not your server is thrashing so that you can take corrective action.
This is a great example of how our own experience of hosting hundreds of customer websites makes Dawn a better product every month. If you're keen to give Dawn a try, head over to our Plans page and get a month's trial for free. We'd love to hear how you get on!
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