I would use a free-form text file at the document root of your server,
with its own little index.
Index is a " "-padded chatroom user name, alongside a "0"-padded string
which is the file pointer.
First 8 bytes of the free-form file (at the pointer) give number of bytes
written for that record. Rest of the record is the chat room text.
You don't need linefeeds and it will be MEGA-fast to access compared
to lumbering your way into SQL for such a simple data recovery task.