Re: [Users] two node ovirt cluster with HA

Hi, Power management makes use of special *dedicated* hardware in order to restart hosts independently of host OS. The engine connects to a power management devices using a *dedicated* network IP address. The engine is capable of rebooting hosts that have entered a non-operational or non-responsive state, The abilities provided by all power management devices are: check status, start, stop and recycle (restart)... In the case of non-responsive host: all of the VMs that are currently running on that host can also become non-responsive. However, the non-responsive host keeps locking the VM hard disk for all VMs it is running. Attempting to start a VM on a different host and assign the second host write privileges for the virtual machine hard disk image can cause data corruption.

Rebooting allows the engine to assume that the lock on a VM hard disk image has been released. The engine can know for sure that the problematic host has been rebooted via the power management device and then it can start a VM from the problematic host on another host without risking data corruption. Important note: A virtual machine that has been marked highly-available can not be safely started on a different host without the certainty that doing so will not cause data corruption. N-joy, --Tareq On 01/27/2014 02:05 PM, Dafna Ron wrote: > I am adding Tareq for the Power Management implementation. > > Dafna > > > On 01/27/2014 11:48 AM, Karli Sjöberg wrote: >> On Mon, 2014-01-27 at 11:11 +0000, Dafna Ron wrote: >>> Powering off the host will never trigger vm migration. >>> As far as engine is concerned it just lost connection to the host, but >>> has no way of telling if the host is down or if a router is down. >> Can´t it at least check with power management if the Host status is down >> first? >> >> I mean, if the network is down there will be no response from either PM >> or Host. But if PM is up and can tell you that the Host is down, sounds >> rather clear cut to me... >> >> Seems to me the VM's would be restarted sooner if the flow was altered >> to first check with PM if it´s a network or Host issue, and if Host >> issue, immediately restart VM's on another Host, instead of waiting for >> a potentially problematic Host to boot up eventually. >> >> /K >> >>> since vm's can continue running on the host even if engine has no >>> access >>> to it, starting the vm's on the second host can cause split brain and >>> data corruption. >>> >>> The way that the engine knows what's going on is by sending heath check >>> queries to the vdsm. >>> Power management will try to reboot a host when the health checks to >>> vdsm will not be answered. >>> So... if engine gets no reply and has no way of rebooting the host, the >>> host status will be changed to Non-Responsive and the vm's will be >>> unknown because engine has no way of knowing what's happening with the >>> vm's. >>> Since reboot of the host will kill the vm's running on it - this will >>> never cause any vm migration but... along with the High-Availability vm >>> feature, you will be able to have some of the vm's re-started on the >>> second host after the host reboot (and that is only if Power Management >>> was confirmed as successful). >>> >>> VM migration is only triggered when: >>> 1. Cluster configuration states that the vm should be migrated in case >>> of failure >>> 2. Engine has access to the host - so the failure is on the storage >>> side >>> and not the host side. >>> 3. the vms are not actively writing (although there might be a new RFE >>> for it). >>> >>> hope this clears things up >>> >>> Dafna >>> >>> >>> >>> On 01/27/2014 10:11 AM, Andrew Lau wrote: >>>> Hi, >>>> >>>> Have you got power management enabled? >>>> >>>> That's the fencing feature required for the engine to ensure that the >>>> host is actually offline. It won't resume any other VMs to prevent >>>> potential VM corruption (eg. VM running on multiple hosts). >>>> >>>> Andrew. >>>> >>>> On Jan 27, 2014 5:12 PM, "Jaison peter" <urotrip2(a)gmail.com >>>> <mailto:urotrip2@gmail.com>> wrote: >>>> >>>> Hi all , >>>> >>>> I was setting a two node ovirt cluster with ovirt engine on >>>> seperate node . I completed the configuration and tested VM live >>>> migrations with out any issues . Then for checking cluster HA I >>>> powered down one host and expected vms running on that host to be >>>> migrated to the other one . But nothing happened , Engine >>>> detected >>>> host as un-rechable and marked it as non-operational and vm >>>> ran on >>>> that host went to 'unknown state' . Is that not possible to setup >>>> a fully HA ovirt cluster with two nodes ? or else is that my >>>> configuration problem ? please advice . >>>> >>>> Thanks & Regards >>>> >>>> Alex >>>> >>>> _______________________________________________ >>>> Users mailing list >>>> Users(a)ovirt.org <mailto:Users@ovirt.org> >>>> http://lists.ovirt.org/mailman/listinfo/users >>>> >>>> >>>> >>>> _______________________________________________ >>>> Users mailing list >>>> Users(a)ovirt.org >>>> http://lists.ovirt.org/mailman/listinfo/users >>> >>> -- >>> Dafna Ron >>> _______________________________________________ >>> Users mailing list >>> Users(a)ovirt.org >>> http://lists.ovirt.org/mailman/listinfo/users >> >> > >