What is happening is that you are writing to the same file that the webserver has an open file handle for. AFAIK, the inodes do not change, you are clobbering the same file while it is opened by the webserver. This is dangerous and poorly designed. What you want is log rotation. Most webservers provide a method to rotate the logs. Apache has apachectl graceful. I recommend that you look into this method to rather than clobbering open files out from under your web server processes.

HTH