somthing about security

[Big Z]

dear all.

       I have installed ubuntu 7.10, and the "encrypted lvm" feature
is interesting.
I have several questions about security:

(1) how about the "encrypted lvm" performance compares with normal lvm?
does it slowdown i/o performance?
(2) Could we disable the encrypted once specially? (such as follow
situation: we want to reboot our remote server,but we don't want to go
to the far datacenter to input encrypted-boot passphrase..we only want
this time the reboot not need input passphrase)

btw: how to disable single-user mode in linux? Is it possible?
somebody said that we could use grub password as alternative, but it
has the same problem, how do we reboot server remotely?

many thanks.

-- 
Best Regards!