A domU VM will time drift out of sync with dom0 and other domUs. Dom0 is actually a paravirtualized guest VM itself and its main purpose is as a management console. Therefore, other VMs running guest operating systems will not have time synced with dom0because they do not get time from dom0 as might be anticipated. Individual guest VMs will drift independently from their startup time for many reasons including individual guest operating system time keeping, type of time calls made by the guest operating system, load running on the guest operating system, and whether the guest is paravirtualized or fully virtualized. Individual guests may drift forward or backward in time independent of dom0 as they do not get time from dom0. Messages such as "time went backwards" may be posted by the Xen hypervisor when a VM is launched.
Source
RETAIN tip: H193588
Symptom
A domU VM will time drift out of sync with dom0 and other domUs. Dom0 is actually a paravirtualized guest VM itself and its main purpose is as a management console. Therefore, other VMs running guest operating systems will not have time synced with dom0 because they do not get time from dom0 as might be anticipated.
Individual guest VMs will drift independently from their startup time for many reasons including individual guest operating system time keeping, type of time calls made by the guest operating system, load running on the guest operating system, and whether the guest is paravirtualized or fully virtualized.
Individual guests may drift forward or backward in time independent of dom0 as they do not get time from dom0.
Messages such as "time went backwards" may be posted by the Xen hypervisor when a VM is launched.
Affected configurations
This tip is not hardware specific.
This tip is not option specific.
The system is configured with at least one of the following:
- Red Hat Enterprise Linux 5, any update
- SuSE Linux Enterprise Server 10, any Service Pack
- SuSE Linux Enterprise Server 10 for AMD64/EM64T, any service pack
Note: This does not imply that the network operating system will work under all combinations of hardware and software.
Please see the compatibility page for more information:
Additional information
This is a generic issue inherent to virtualization that can be worked around by using a standardized time source.
If it is desired or necessary for a particular usage model to have all VMs have a synchronized time then all VMs must be setup to use a standardized time source external to their runtime environment exactly like the bare metal systems they are replacing would have required.
The external time system used and its setup are customer environment specific. It is expected that anything that worked on bare metal systems should work for the VMs that are replacing them.
Standardized network time protocols such as NTP have been proven to work.
Example:
Each guest operating system in each VM is setup to use NTP via a daemon. When the guest is first booted it should make the initial time sync by running a onetime process such as ntpdate. Afterwards, the NTP daemon maintains time during runtime.
Depending on your application, for paravirtualized guest VMs including dom0, it may also be necessary to toggle the wall clock time from the default setting inside the individual paravirtualized guest. From a command prompt as root inside the individual paravirtualized guest:
| |
echo 1 > /proc/sys/xen/independent_wallclock |
That will set the paravirtualized vm to use its own clock which in this example is ntp.
To change the setting permanently in the vm, edit the /etc/sysctl.conf file inside the vm and add/change:
| |
xen.independent_wallclock = 1 |
Guests do not get time from dom0 since it is a paravirtualized guest running inside of a VM. Therefore, they will not keep time synced with dom0 or any other guest VM that is running.
Setting all guest operating systems in each VM to use an external precision time source, such as NTP, will allow all VMs to maintain synced time independent of the guest operating system and workload.
System x:Red Hat Enterprise Linux 5
System x:Red Hat Enterprise Linux 5 x86-64
System x:SUSE Linux Enterprise Server 10
System x:SUSE Linux Enterprise Server 10 x86-64
[{"Type":"HW","Business Unit":{"code":"BU016","label":"Multiple Vendor Support"},"Product":{"code":"HW312","label":"System x->System x3800"},"Platform":[{"code":"PF042","label":"Caldera"},{"code":"PF047","label":"SurePOS"}],"Line of Business":{"code":"","label":""}},{"Type":"HW","Business Unit":{"code":"BU016","label":"Multiple Vendor Support"},"Product":{"code":"HW313","label":"System x->System x3650 T"},"Platform":[{"code":"PF042","label":"Caldera"},{"code":"PF047","label":"SurePOS"}],"Line of Business":{"code":"","label":""}},{"Type":"HW","Business Unit":{"code":"BU016","label":"Multiple Vendor Support"},"Product":{"code":"HW314","label":"System x->System x3850"},"Platform":[{"code":"PF042","label":"Caldera"},{"code":"PF047","label":"SurePOS"}],"Line of Business":{"code":"","label":""}},{"Type":"HW","Business Unit":{"code":"BU016","label":"Multiple Vendor Support"},"Product":{"code":"HW315","label":"System x->System x3950"},"Platform":[{"code":"PF042","label":"Caldera"},{"code":"PF047","label":"SurePOS"}],"Line of Business":{"code":"","label":""}},{"Type":"HW","Business Unit":{"code":"BU054","label":"Systems w\/TPS"},"Product":{"code":"HW316","label":"System x->System x3950 E"},"Platform":[{"code":"PF042","label":"Caldera"},{"code":"PF047","label":"SurePOS"}],"Line of Business":{"code":"","label":""}},{"Type":"HW","Business Unit":{"code":"BU016","label":"Multiple Vendor Support"},"Product":{"code":"HW317","label":"System x->System x3500"},"Platform":[{"code":"PF042","label":"Caldera"},{"code":"PF047","label":"SurePOS"}],"Line of Business":{"code":"","label":""}},{"Type":"HW","Business Unit":{"code":"BU016","label":"Multiple Vendor Support"},"Product":{"code":"HW318","label":"System x->System x3550"},"Platform":[{"code":"PF042","label":"Caldera"},{"code":"PF047","label":"SurePOS"}],"Line of Business":{"code":"","label":""}},{"Type":"HW","Business Unit":{"code":"BU016","label":"Multiple Vendor Support"},"Product":{"code":"HW319","label":"System x->System x3650"},"Platform":[{"code":"PF042","label":"Caldera"},{"code":"PF047","label":"SurePOS"}],"Line of Business":{"code":"","label":""}},{"Type":"HW","Business Unit":{"code":"BU016","label":"Multiple Vendor Support"},"Product":{"code":"HW321","label":"System x->System x3400"},"Platform":[{"code":"PF042","label":"Caldera"},{"code":"PF047","label":"SurePOS"}],"Line of Business":{"code":"","label":""}},{"Type":"HW","Business Unit":{"code":"BU016","label":"Multiple Vendor Support"},"Product":{"code":"HW322","label":"System x->System x3455"},"Platform":[{"code":"PF042","label":"Caldera"},{"code":"PF047","label":"SurePOS"}],"Line of Business":{"code":"","label":""}},{"Type":"HW","Business Unit":{"code":"BU016","label":"Multiple Vendor Support"},"Product":{"code":"HW323","label":"System x->System x3655"},"Platform":[{"code":"PF042","label":"Caldera"},{"code":"PF047","label":"SurePOS"}],"Line of Business":{"code":"","label":""}},{"Type":"HW","Business Unit":{"code":"BU016","label":"Multiple Vendor Support"},"Product":{"code":"HW324","label":"System x->System x3755"},"Platform":[{"code":"PF042","label":"Caldera"},{"code":"PF047","label":"SurePOS"}],"Line of Business":{"code":"","label":""}},{"Type":"HW","Business Unit":{"code":"BU016","label":"Multiple Vendor Support"},"Product":{"code":"HW328","label":"System x->System x3105"},"Platform":[{"code":"PF042","label":"Caldera"},{"code":"PF047","label":"SurePOS"}],"Line of Business":{"code":"LOB26","label":"Storage"}},{"Type":"HW","Business Unit":{"code":"BU016","label":"Multiple Vendor Support"},"Product":{"code":"HW329","label":"System x->System x3200"},"Platform":[{"code":"PF042","label":"Caldera"},{"code":"PF047","label":"SurePOS"}],"Line of Business":{"code":"","label":""}},{"Type":"HW","Business Unit":{"code":"BU016","label":"Multiple Vendor Support"},"Product":{"code":"HW331","label":"System x->System x3250"},"Platform":[{"code":"PF042","label":"Caldera"},{"code":"PF047","label":"SurePOS"}],"Line of Business":{"code":"","label":""}},{"Type":"HW","Business Unit":{"code":"BU016","label":"Multiple Vendor Support"},"Product":{"code":"HW332","label":"System x->System x3850 M2"},"Platform":[{"code":"PF042","label":"Caldera"},{"code":"PF047","label":"SurePOS"}],"Line of Business":{"code":"","label":""}},{"Type":"HW","Business Unit":{"code":"BU016","label":"Multiple Vendor Support"},"Product":{"code":"HW333","label":"System x->System x3950 M2"},"Platform":[{"code":"PF042","label":"Caldera"},{"code":"PF047","label":"SurePOS"}],"Line of Business":{"code":"","label":""}},{"Type":"HW","Business Unit":{"code":"BU016","label":"Multiple Vendor Support"},"Product":{"code":"HW334","label":"System x->System x3200 M2"},"Platform":[{"code":"PF042","label":"Caldera"},{"code":"PF047","label":"SurePOS"}],"Line of Business":{"code":"","label":""}},{"Type":"HW","Business Unit":{"code":"BU016","label":"Multiple Vendor Support"},"Product":{"code":"HW335","label":"System x->System x3250 M2"},"Platform":[{"code":"PF042","label":"Caldera"},{"code":"PF047","label":"SurePOS"}],"Line of Business":{"code":"","label":""}},{"Type":"HW","Business Unit":{"code":"BU016","label":"Multiple Vendor Support"},"Product":{"code":"HW336","label":"System x->System x3350"},"Platform":[{"code":"PF042","label":"Caldera"},{"code":"PF047","label":"SurePOS"}],"Line of Business":{"code":"","label":""}},{"Type":"HW","Business Unit":{"code":"BU016","label":"Multiple Vendor Support"},"Product":{"code":"HW337","label":"System x->System x3100"},"Platform":[{"code":"PF042","label":"Caldera"},{"code":"PF047","label":"SurePOS"}],"Line of Business":{"code":"","label":""}},{"Type":"HW","Business Unit":{"code":"BU016","label":"Multiple Vendor Support"},"Product":{"code":"HW338","label":"System x->System x3450"},"Platform":[{"code":"PF042","label":"Caldera"},{"code":"PF047","label":"SurePOS"}],"Line of Business":{"code":"","label":""}},{"Type":"HW","Business Unit":{"code":"BU016","label":"Multiple Vendor Support"},"Product":{"code":"HW339","label":"System x->System x3610"},"Platform":[{"code":"PF042","label":"Caldera"},{"code":"PF047","label":"SurePOS"}],"Line of Business":{"code":"","label":""}},{"Type":"HW","Business Unit":{"code":"BU016","label":"Multiple Vendor Support"},"Product":{"code":"HWXA1","label":"System x->System x3650 NAS"},"Platform":[{"code":"PF042","label":"Caldera"},{"code":"PF047","label":"SurePOS"}],"Line of Business":{"code":"","label":""}}]