I am having this issue. I just upgraded to 4.5.7 on Centos 5. I have not had any stats gathered since the upgrade. zimbra.log shows that the service is not running. I found some errors in the /opt/zimbra/zmstat/zmstat.out. not sure what they mean.
Code:
No iostat installed on this host at /opt/zimbra/libexec/zmstat-io line 37.
No iostat installed on this host at /opt/zimbra/libexec/zmstat-io line 37.
Warning: Not possible to monitor process stats