Re: 9 TiB vm memory creation

[Igor Mammedov]

On Mon, 14 Feb 2022 10:54:22 +0530 (IST)
Ani Sinha <ani@anisinha.ca> wrote:

> Hi Igor:
> 
> I failed to spawn a 9 Tib VM. The max I could do was a 2 TiB vm on my
> system with the following commandline before either the system
> destabilized or the OOM killed killed qemu
> 
> -m 2T,maxmem=9T,slots=1 \
> -object 
> memory-backend-file,id=mem0,size=2T,mem-path=/data/temp/memfile,prealloc=off \
> -machine memory-backend=mem0 \
> -chardev file,path=/tmp/debugcon2.txt,id=debugcon \
> -device isa-debugcon,iobase=0x402,chardev=debugcon \
> 
> I have attached the debugcon output from 2 TiB vm.
> Is there any other commandline parameters or options I should try?
> 
> thanks
> ani

$ truncate -s 9T 9tb_sparse_disk.img
$ qemu-system-x86_64 -m 9T \
  -object 
memory-backend-file,id=mem0,size=9T,mem-path=9tb_sparse_disk.img,prealloc=off,share=on
 \
  -machine memory-backend=mem0

works for me till GRUB menu, with sufficient guest kernel
persuasion (i.e. CLI limit ram size to something reasonable) you can boot linux
guest on it and inspect SMBIOS tables comfortably.


With KVM enabled it bails out with:
   qemu-system-x86_64: kvm_set_user_memory_region: KVM_SET_USER_MEMORY_REGION 
failed, slot=1, start=0x100000000, size=0x8ff40000000: Invalid argument

all of that on a host with 32G of RAM/no swap.