From your message I am not 100% on your issue. Is the db2diag.log what filled your file system? If so move the current file to another location and that will free up the space. Depending on your version of db2 you can have this done automatically. If you filled your transaction log directory do not delete the logs. First check how many primary/secondary logs you can allocated and make sure you have sufficient space. It is best practices to isolate these to their own file system. If you do not have sufficient space you can either increase the file system or decrease the number/size of logs that db2 can allocate. This will of course require an outage.