Xen virtual machines and ntp

[ESGLinux]

Hi,

I´m running a  Xen server with several virtual machines on it. I have a
problem with the time of the machines.
They unsynchronize very fast (in an hour several seconds). I try to use ntpd
to syncronize them but It doesn´t work.

I have set up a local ntp server with this ntp.conf:

#grep  ^[^#] /etc/ntp.conf
restrict default kod nomodify notrap
restrict 192.168.1.0 mask 255.255.255.0 nomodify notrap
server 1.rhel.pool.ntp.org
server 2.rhel.pool.ntp.org
server 127.127.1.0
fudge   127.127.1.0 stratum 10
driftfile /var/lib/ntp/drift
keys /etc/ntp/keys
restrict 1.rhel.pool.ntp.org mask 255.255.255.255 nomodify notrap noquery
restrict 2.rhel.pool.ntp.org mask 255.255.255.255 nomodify notrap noquery
server hora.roa.es
restrict hora.roa.es mask 255.255.255.255 nomodify notrap noquery

and the clients like this:

restrict default kod nomodify notrap nopeer noquery
tinker step 0
restrict 127.0.0.1
restrict -6 ::1
server 192.168.1.1
server 0.rhel.pool.ntp.org
server 1.rhel.pool.ntp.org
server 2.rhel.pool.ntp.org
driftfile /var/lib/ntp/drift
keys /etc/ntp/keys
restrict 0.rhel.pool.ntp.org mask 255.255.255.255 nomodify notrap noquery
restrict 1.rhel.pool.ntp.org mask 255.255.255.255 nomodify notrap noquery
restrict 2.rhel.pool.ntp.org mask 255.255.255.255 nomodify notrap noquery
restrict 192.168.1.1 mask 255.255.255.255 nomodify notrap noquery

192.168.1.1 is the first machine, the one I want to be the ntp local server.

the param tinker step 0 I get from this
http://wiki.dovecot.org/TimeMovedBackwards
My real problem began with dovecot, because if the machine is not well
syncronized the dovecot kills itself ( dovecot: Time just moved backwards by
8 seconds. This might cause a lot of problems, so I'll just kill myself now.
http://wiki.dovecot.org/TimeMovedBackwards)


so, I have two questions:

1 - Why my virtual machines unsynchronize so quickly?
2 - What am I doing wrong with ntpd?

thanks in advance

ESG