I have a web server that has been running pretty smoothly for years. Twice recently it has gone down for no apparent reason - the monit logs don't show any particularly high anything going on. CPU usage is low, network out is 0, network in is low. Logs don't show any unusual requests around that time for my PHP or Django applications, or anything weird in the kernel log (that I can tell). Plenty of free disk space. I tried to ssh in, and was also unable to get in from my VPS providers Ajax console. I had to reboot the system.
Every system log file open at the time has garbled stuff at that time some - it shows in less as '^@^@^@^@^@^@'. While the system is crashed or whatever, nothing seems to be happening (no cronjobs, for instance, no syslog entries).
When this happened a second time, right in a log file I was writing from PHP at that time, there's a there's a chunk of text apparently from another file on disk - an SQL backup I have somewhere else on disk, from a couple of weeks ago (it has the timestamp written by mysqldump). How did that end up in my log file?
The only recent changes I can think of, other than minor changes to my PHP application code, are that I moved from a 4 GB ram VPS to a 2 GB, and may have been moved to a different physical machine.
Any ideas what might be going on here?