On Sat, Dec 21, 2013 at 11:56 PM, Grégoire Leroy < [email protected]> wrote:
> Hello, > > > > If you disable quorum then you won't have the issue of "read only" > when > > > > > >> you lose a host, but you > won't have protection from split brain (if > > >> your > > >> two hosts lose network connectivity). VMs will > > >> keep writing to the hosts, as you have the gluster server and client > on > > >> the same host this is > > >> inevitable. > > > > Ok, I get the problem caused by disabling the quorum. So, what if while > > > I've two hosts the lack of HA is not so dramatic but will be necessary > > > when > > > > > I'll have more hosts ? (3 or 4). Here is the scenario I would like to > have > : > > Quorum generally requires 3 hosts, I believe the default configuration > when > > you press "Optimize for virt store" will require a minimum of 2 bricks > > connected before writing is allowed. > > Ok, if I understand, the quorum thing is very specific to gluster (bricks) > and > not to ovirt (hosts). So, maybe what I need is just another gluster server > with very few space on a dummy VM (not hosted by a ovirt host but outside > of > my cluster) to add as a brick. It wouldn't be use at all, just to check > connectivity > > Then, if a host lose connectivity, it can't join neither the real gluster > server nor the "dummy" one and so, doesn't run VM. The other one, which is > able to join the dummy one becomes the SPM (the dummy wouldn't have vdsm > server, so it couldn't become) and runs VM. > > Maybe by this way could I have HA with two hosts, right ? Is there a > reason it > shouldn't work ? I guess this would work as quroum is based on how many peers are in the cluster. Actually quite a good idea and I'd love to hear from you on how it goes. I'd be interested to see how gluster will work with this though, I assume it has to be apart of the volume. If you're doing distribute-replicate I think this "dummy" VM will need to hold hold the full replicated data? cluster.server-quorum-ratio - this is % > 50. If the volume is not set with any ratio the equation for quorum is: active_peer_count > 50% of all peers in cluster. But when the percentage (P) is specified the equation for quorum is active_peer_count >= P % of all the befriended peers in cluster. > > 1) I have two hosts : HOSTA and HOSTB. They have glusterfs bricks > > > configured as Distribute Replicated and data is replicated. > > > => For now, I'm totally ok with the fact that if a node fails, then VM > on > > > this hosts are stopped and unreachable. However, I would like that if a > > > node fails, the DC keeps running so that VM on the other hosts are not > > > stopped and a human intervention make possible to start the VM on the > > > other > > > host. Would it be possible without disabling the quorum ? > > > > For the 2 host scenario, disable quorum will allow you to do this. > > Unfortunately, not for all cases. If the network interface used by > glusterfs > to reach each other falls, I get the following behaviour : > > 1) HOSTB, on which the VM run, detect that HOSTA's brick is unreachable. > So it > keeps running. Fine. > 2) HOSTA detects that HOSTB's brick is unreachable. So it starts to run > the VM > => Split brain. If the network interfaces not used for management of the > cluster but for VM are OK, I'm going to have a split network. > 3) Conclusion, the fall of HOSTA has impact on the VM of HOSTB > > Does this scenario seem correct to you, or have I miss something ? Maybe > power > management could avoid this issue. > Yes you'll need the power management which they call "fencing", so it will ensure that the host which has dropped from the cluster is sent for a reboot thus making any VMs running on it be shut off immediately and ready to be brought up on another ovirt host. > > > > 2) In few months, I'll add two other hosts to the glusterfs volum. > Their > > > bricks will be replicated. > > > => At that time, I would like to be able to make evolve my architecture > > > (without shut my VM and export/import them on a new cluster) so that > if a > > > node fails, VM on this host start to run on the other host of the same > > > brick (without manual intervention). > > > > Later on you just enable quorum, it's only a setting in the gluster > volume. > > gluster volume set DATA cluster.quorum-type auto > > Thanks you, > Regards, > Grégoire Leroy > >
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