How good is the paravirtualized clock?
I need to provide good time to our network infrastructure, and users and am wondering if it makes sense to have RHEV VMs as stratum-2 NTP-servers, connected to various stratum-1 NTP appliances. Will virtual machines keep good enough time to be used as NTP clocks for other servers?
I've noticed on serverfault.com they're quite negative to running NTP-servers on VMs:
http://serverfault.com/questions/106501/what-are-the-limits-of-running-ntp-servers-in-virtual-machines?lq=1
is there any truth to this, or just old wisdom from the horrible vmware days? How good will the timekeeping be? Within 30-100ms as suggested on serverfault, or are there reasons to expect the KVM paravirtualized clock to perform as well as a physical clock?
Will VM migration be problematic for NTP? Maybe best to not migrate these VMs?
KVM timekeeping reference document: http://lxr.linux.no/#linux+v3.8/Documentation/virtual/kvm/timekeeping.txt
Original kvmclock documentation: https://lkml.org/lkml/2010/4/15/355
Responses
Red Hat
25369 points
Hi Jan-Frode, did you see this related, recent discussion https://access.redhat.com/discussion/kvm-clock ?
Guru
6827 points
Since the time David posted the link above, the Red Hat Discussion area has changed and all links subsequently changed. The new URL for the "kvm-clock" discussion he cites above is at this location https://access.redhat.com/discussions/686523
The link posted by David, gives me problems, because of incorrect redirection. For diagnosis:
$ lwp-request -m HEAD -P -H 'User-Agent:' https://access.redhat.com/discussion/kvm-clock
301 Moved Permanently
Connection: close
Date: Mon, 01 Feb 2016 17:58:35 GMT
Location: https://access.redhat.com/discussion/kvm-clock
Server: BigIP
Content-Length: 0
Client-Date: Mon, 01 Feb 2016 17:58:35 GMT
Client-Peer: 23.78.32.159:443
Client-Response-Num: 1
Client-SSL-Cert-Issuer: /C=NL/L=Amsterdam/O=Verizon Enterprise Solutions/OU=Cybertrust/CN=Verizon Akamai SureServer CA G14-SHA2
Client-SSL-Cert-Subject: /C=US/ST=NC/L=Raleigh/O=Red Hat, Inc./OU=Web Operations/CN=access.redhat.com
Client-SSL-Cipher: AES256-SHA
Client-SSL-Socket-Class: IO::Socket::SSL
Client-Warning: Redirect loop detected (max_redirect = 7)
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