> Hi, > >I'd like to ask around if someone does run oVirt > with NFS backed Storage provided by simple servers (no SAN or NAS) > and what your experience is so far?
We are running OVirt (still in test phase) on three self build equal Ubuntu NFS servers over Infiniband (50 Euros for a ConnectX card + 50 Euros a cable + 400 Euros a switch). Due to massive bandwidth the bottleneck are the SATA disks and the RAID controller. I fixed some info at http://www.ovirt.org/Infiniband Our soon to be replaced ESX infrastucture uses the same platform. > In particular I'm interested what happens if there is a connection > loss to the NFS-Volume. You can be sure that the applications in your VMs will get stuck. They usually flush their write caches in regular intervals. This is directly translated into a NFS write on the hypervisor. At least it should be configured that way otherwise you could loose data in case of a crash. E.g. VM thinks data is persisent, but it was only cached in the hypervisor RAM. > How does this affect running vms and the compute nodes they run on? The hypervisor has no problems at all (except you run a df command) and the VMs usually will continue their operation after NFS recovers. Nevertheless the are lot of timeout settings in the architecute stack that may stop a running application inside the VM. Expect do have a lot of manual cleanup after a NFS failure. > I suspect they would first write their changes to RAM instead of virtual HDD. > But once the RAM is full, does just the vm become unresponsive or > does the whole compute node die? That would be bad practice. See above. > I couldn't test this yet myself, but my limited experience with > NFS-Servers tells me, that they become unresponsive if they are under > heavy load and I'd like to know how this affects the vms and > computenode. Defining heavy load is basically some kind of I/O calculation. A simple example from our setup. - Each NFS server has a RAID6 consisting of 14 SATA disks (7.2K) - The top-loaded NFS server runs a minimum of 140 8K I/Os per second - Assuming a random pattern we speak of 140 Read-Modify-Write cycles - That translates to roughly to 6*140=840 I/Os per second. - The 14 SATA disks offer a maximum of 14*90=1260 I/Os per second - No wonder that the NFS server gives usually a 20% wait I/O usage. > Thanks! > PS: Bonus question: Does someone utilize the NFS-Servers also as > computenodes ? We spearated that stricly. Otherwise we would go with a Gluster FS. Markus P.S. We leave VMWare because live storage migration of OVirt VMs gives us the possibility to reduce the complexity on the storage side.
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