[Qemu-devel] [Bug 1769053] Re: Cannot start a guest with more than 1TB of RAM

[Dan Streetman]

You don't need a >1TB host to spin up a >1TB guest.  Unless you're using
pci passthru (and/or SRIOV), or something else that requires qemu to
alloc and pin all guest mem, you can simply overcommit; normal guests
don't require mem pre-allocation or pinning.

On your host do this to allow overcommitting such a large amount (this
allows 16T but can be adjusted as needed):

$ echo $[ 16 * 1024 * 1024 * 1024 ] | sudo tee /proc/sys/vm/overcommit_kbytes 
17179869184
$ echo 1 | sudo tee /proc/sys/vm/overcommit_memory 
1


Then just virsh edit your guest to use >1TB, e.g.:

  <memory unit='GiB'>1500</memory>


And of course, stop and restart the guest to pick up the xml change.

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https://bugs.launchpad.net/bugs/1769053

Title:
  Cannot start a guest with more than 1TB of RAM

Status in QEMU:
  Incomplete
Status in qemu package in Ubuntu:
  Incomplete

Bug description:
  Attempting to start a KVM guest with more than 1TB of RAM fails.

  It looks like we might need some extra patches:
  https://lists.gnu.org/archive/html/qemu-discuss/2017-12/msg00005.html

  ProblemType: Bug
  DistroRelease: Ubuntu 18.04
  Package: qemu-system-x86 1:2.11+dfsg-1ubuntu7
  ProcVersionSignature: Ubuntu 4.15.0-20.21-generic 4.15.17
  Uname: Linux 4.15.0-20-generic x86_64
  ApportVersion: 2.20.9-0ubuntu7
  Architecture: amd64
  CurrentDesktop: Unity:Unity7:ubuntu
  Date: Fri May  4 16:21:14 2018
  InstallationDate: Installed on 2017-04-05 (393 days ago)
  InstallationMedia: Ubuntu 16.10 "Yakkety Yak" - Release amd64 (20161012.2)
  MachineType: Dell Inc. XPS 13 9360
  ProcKernelCmdLine: BOOT_IMAGE=/vmlinuz-4.15.0-20-generic 
root=/dev/mapper/ubuntu--vg-root ro quiet splash transparent_hugepage=madvise 
vt.handoff=1
  SourcePackage: qemu
  UpgradeStatus: Upgraded to bionic on 2018-04-30 (3 days ago)
  dmi.bios.date: 02/26/2018
  dmi.bios.vendor: Dell Inc.
  dmi.bios.version: 2.6.2
  dmi.board.name: 0PF86Y
  dmi.board.vendor: Dell Inc.
  dmi.board.version: A00
  dmi.chassis.type: 9
  dmi.chassis.vendor: Dell Inc.
  dmi.modalias: 
dmi:bvnDellInc.:bvr2.6.2:bd02/26/2018:svnDellInc.:pnXPS139360:pvr:rvnDellInc.:rn0PF86Y:rvrA00:cvnDellInc.:ct9:cvr:
  dmi.product.family: XPS
  dmi.product.name: XPS 13 9360
  dmi.sys.vendor: Dell Inc.

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