Lately I have noticed a lot of the following exceptions in our client’s Sitecore log:
Exception: SolrNet.Exceptions.SolrConnectionException Message: The operation has timed out
This started to happen as our dedicated processing server is going through a few months of data as it was not running properly for a while.
I have spent some time trying to tweak the Solr server’s parameters such as the request time out without seeing any improvement in regards to the number of logged time out exceptions.
Eventually I approached Sitecore support and they told me that this is a known bug in Sitecore 8.1 which has been fixed for 8.2.
If you are running 8.1 same as us and you experience this problems, I recommend you reach out to Sitecore support as they have a patch for each of their support DI containers which will allow you to configure the Solr time out in the settings.
I hope this helps others to save the time investigating this issue.