Fastest Way To Update 20 + Million Records

Hello,
What is the fastest way to update 20million records in our database.
I have tried to do a simple update statement like this:
update trail_log with (tablockx, holdlock)
set trail_log .entry_by = users.user_identity
from users
where trail_log.entry_by = users.user_id

but it take 10 plus hours to run since it cannot commit the transactions until the very end. So was was thinking that I need to commit in batch like after 50K but that is slow as well.
Set rowcount 50000
Declare @rc int
Set @rc=50000
While @rc=50000
Begin
Begin Transaction
update trail_log With (tablockx, holdlock)
set trail_log.entry_by = users.user_identity
from users
where trail_log.entry_by = users.user_id
and trail_log.entry_by not like '%[0-9]%'
Select @rc=@@rowcount
--Commit the transaction
Commit
End
go
I have let the above statement run for 1.5 hours and it only update 450000 rows. Any ideas...
Maybe I'm doing it wrong. Please Help!!