We've found some new openvz accounts resported some ICMP packet loss from time to time, and we had a chance to test 1 account yesterday and we found out that it was because it was out of buffer space.

If the vps is not out of resources, make sure you have continuous traffic flow in/out of the vps. Some networking might go to sleep, if you do not have any traffic at all.

One of important limitations is limit of size of unswapped memory allocated to kernel. In this area resides tcpsndbuf. It's summary size of buffers that are used to send data. If the value is low your download speed may be limited and sometimes you are getting error:

105 No buffer space available

You can check your limits by executing the following command:

# grep tcpsndbuf /proc/user_beancounters
tcpsndbuf  174336   1735168   1720320   2703360  2801251

Current buffer usage is 174336 bytes, limit is 1720320 bytes and limit was exceeded 2801251 times (ops!). Why send buffer is the problem here instead of receive buffer? Most of the time HTTP server is sending data – requests are much smaller than HTTP responses.

Solutions for this error:

  • Limit concurrent connections server handles
  • Check if unused connections are closed as soon as possible

Wednesday, March 9, 2016

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