> > > > What we are still thinking about is the point, if it is principally > > a > > good idea to limit CloudStack in its ability to freely and > > automatically > > migrate VMs between all cluster nodes. Is setting > > "enable.ha.storage.migration"=false the intended way to handle a > > setup > > with multiple clusters or is it kind of a dirty hack to circumvent > > disadvantages of our setup? In the latter case we would like to > > know > > that and keep a focus on alternatives and be ready to improve our > > setup > > in the mid-term. > The logic around having multiple primary storage options tied to > clusters is really designed to limit failure domains. Ideally you > want to spread your workloads across different failure domains so > that if you do lose a primary storage system, you still have services > up and running. > We build redundancy into the cluster and the storage attached to the > cluster. We also run multiple clusters within a pod. If you spread > your redundant VMs across multiple clusters (with their own primary > storage), it's easier to absorb a catastrophic storage failure, as > your eggs aren't in one basket.
Thank you for this explanation. > We turn off HA storage migration, as it doesn't make much sense to > us. It assumes the storage is still up, as you obviously can't > migrate a VM to a different primary storage if it's down. If you have > enough hosts in a cluster, you should never run into a situation > where you can't bring all your VMs back up due to host failure. So in > that sense, HA storage migration is a pointless feature if you build > and scale your clusters properly. Indeed, even if you have enough hosts, we ran into that situation due to a bug with multiple datadisks and hvm introduced via https://github. com/apache/cloudstack/pull/792 . As a result acs tries to start the vm on node1,2,3... and so on and fails on all hosts due to the underlying qemu-dm parsing error. Finally it tries to start on nodes of another cluster which subsequently triggers a storage migration. - Stephan
