While upgrading a SharePoint farm to the December Cumulative update, there was an unprecendented growth of the databse log file for a large content database (size ~200GB) during the upgrade process. The database upgrade was failing as there was no space in the log file partition. Also, the log file could not be truncated to free up the space.

This was overcome by changing the Database Recovery model from Full to Simple. Under Full Recovery Model, log files are not truncated untill a log backup has been completed. With Simple recovery model this problem can be avoided as transactions are flushed out automatically till the last committed  point.

Product team recommends that the recovery model for content databases should be set to full. However, during upgrades, it might be a good idea to change it to simple and revert that back to full once the upgrade completes.