Re: [libvirt-users] starting a domain only when you have enough resources

On Tue, Jan 26, 2016 at 12:39 PM, Michal Privoznik <mprivozn(a)redhat.com&gt; wrote: > On 26.01.2016 12:30, Andrei Perietanu wrote: >> Hi all, >> >> I am running KVM on a 3.18 kernel. The system runs and Atom processor > with >> 2Gb RAM. >> >> Using KVM you obviously can over allocate your resources: say you have 4 >> guests each configured with 1GB ram. Running all four at the same time, >> depending on the workload, can crash the system - I get a kernel trace > when >> this happens. >> >> But let's consider a simpler case: one guest with 1.5 Gb RAM, ubuntu > 14.03. >> During the installation the system will again crash. >> The memory statistics (top or proc/meminfo) will show that the FreeMemory >> goes down to 12Mb when this happens - which kind of makes sense > considering >> the host will require some RAM to run. >> >> But the question is: does libvirt offer any way to prevent this from >> happening? >> >> Some way of not allowing the user to start a guest unless you have enough >> free memory. I know how much ram each guest has configured but that is > not >> enough. I need to know how much the system has available, and just > reading >> the free memory statistic does not help much since that is only a > snapshot >> - when running a guest you can have 1gb free now, and 10 mb free 2 min >> later. >> >> Any ideas? > > There is one option I see, use -mem-prealloc. Either you can passthrough > it onto qemu commandline [1] or use locked memoryBacking [2]. I advocate > for the latter though. Not only it will allocate all the memory at qemu > startup it will also lock it so it won't get swapped off. > > Michal > > 1: http://libvirt.org/drvqemu.html#qemucommand > 2: http://libvirt.org/formatdomain.html#elementsMemoryBacking > I tried memoryBacking: I added this to the domain xml: <memoryBacking> <locked/> </memoryBacking> And got an error when attempting to start the vm: memory locking no supported by QEMU binary. Aside from that I don't really understand how this helps solve my issue - if you don't mind going a bit into details I'd appreciate it.