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libvirt: Add reasonable memory_backing_dir default for qemu. #105123

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libvirt: Add reasonable memory_backing_dir default for qemu. #105123

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clefru
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@clefru clefru commented Nov 27, 2020

When VMs are run with "",
qemu creates a backing file to make memory accessible to the guest and
other host processes. Without specifying memory_backing_dir, the
backing file actually lands on disk inside /var/lib/libvirt, which
makes every memory access inside the guest an actual filesystem IO.

This commit provides a reasonable default inside qemuVerbatimConfig.

Motivation for this change
Things done
  • Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
  • Built on platform(s)
    • NixOS
    • macOS
    • other Linux distributions
  • Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
  • Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
  • Tested execution of all binary files (usually in ./result/bin/)
  • Determined the impact on package closure size (by running nix path-info -S before and after)
  • Ensured that relevant documentation is up to date
  • Fits CONTRIBUTING.md.

@clefru
libvirt: Add reasonable memory_backing_dir default for qemu.
6e48d4f 
When VMs are run with "<memoryBacking><access mode='shared'/></memoryBacking>",
qemu creates a backing file to make memory accessible to the guest an
other host processes. Without specifying memory_backing_dir, the
backing file actually lands on disk inside /var/lib/libvirt, which
makes every memory access inside the guest and actual filesystem IO.

This commit provides a reasonable default inside qemuVerbatimConfig.
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clefru commented Feb 12, 2021

@fpletz @globin friendly ping for maintainers of libvirt.

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clefru commented May 13, 2021

Friendly ping.

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clefru commented Jun 14, 2021

@fpletz @globin friendly ping for maintainers of libvirt.

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teto commented Jun 14, 2021

I would be wary of changing libvirt defaults since they have been chosen for a reason. Nevertheless the option looks like something most people want so I would move it to the example section of qemuVerbatimConfig.

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clefru commented Jun 30, 2021 •
edited

I would be wary of changing libvirt defaults since they have been chosen for a reason. Nevertheless the option looks like something most people want so I would move it to the example section of qemuVerbatimConfig.

I don't find this default reasonable, because /dev/shm is exactly the right place for creating large backing files for shared memory. /var/lib/libvirt isn't, as this is nearly always going to land on disk.

But maybe I can get upstream to change it. I'll drop this PR, as going for upstream seems like a better use of time. Thank you for the feedback, Matthieu.

@clefru clefru closed this Jun 30, 2021
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6.topic: nixos 8.has: module (update) 10.rebuild-darwin: 0 10.rebuild-linux: 1-10 10.rebuild-linux: 1
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