hostname and hostname -f not same value

[mtuser]

following this guide

hostname = ns1.itemalls.com
hostname -f = ns1.itemalls.com

but after reboot it's changed to

hostname = ns1
hostname -f = ns1.itemalls.com

is it correct?

[till]

Is this a vserver with e.g. openvz?

[Parsec]

It's the correct procedure. hostname -f, the -f means the fully qualified domain name of the server. Without the -f you just get the hostname, which is the name of the server (without the domain name).

[mtuser]

One debian 6 64bit it's same value.
but one in debian 32bit VPS (Openvz) it's not same.
why?

[Parsec]

Most probably depends on what you typed when asked originally. What is stored in /etc/hostname and initiated at boot by /etc/init.d/hostname.sh

Take a look in those files and also view the manual for hostname (man hostname) and you will understand how it works.