The following command is the efficient one to delete/truncate all the entries from a file which is being actively used:
truncate -s 0 filename
Refer: http://linux.die.net/man/1/truncate
On the other hand, truncate would likely be slower than > filename when run from a script since running the truncate command requires the system to open the executable, load it, and the run it.
Suneil Rai's Weblog 