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Storage and persistence

The root filesystem (/) and /tmp are tmpfs mounts that are wiped on every reboot. Anything you want to keep should be kept on a persitent volume like $HOME. Some modules provide persistent volumes and store them on the host as QEMU images under /var/lib/microvms/<name>/<image>.img

See options reference for the available volumes and sizes.

Nix store share

The host’s /nix/store is shared into the guest as a read-only filesystem, with a writable overlay on top of it. Set mariner.storage.readOnlyStoreShare to select the protocol for sharing the host’s store or set it to null if you don’t want to share the store.

Volume sizes

Volume images are sparse: a volume with an apparent size of 32GB only allocates the space that’s actually used. The allocated size will grow over time as you create and delete files, but a scheduled fstrim.service in the VM reclaims the freed blocks back to the host.

The size is only applied when the image is first created. Changing a SizeMiB option later doesn’t resize the volume.

$ du -hs --apparent-size  /var/lib/microvms/vm/*
32G     /var/lib/microvms/vm/nix-store.img
8.0G    /var/lib/microvms/vm/persist.img

$ du -hs  /var/lib/microvms/vm/*
1.4G    /var/lib/microvms/vm/nix-store.img
613M    /var/lib/microvms/vm/persist.img

Resizing a volume

You can resize the volumes after they are created. Make sure to stop the virtual machine before doing filesystem operations otherwise you will corrupt the volume and lose data. It’s recommended to only grow the images. For shrinking you have to first shrink the file system size before running qemu-img resize

sudo systemctl stop microvm@<name>.service

sudo qemu-img resize --format raw /var/lib/microvms/<name>/<image>.img <new-size>
sudo e2fsck -f /var/lib/microvms/<name>/<image>.img
sudo resize2fs /var/lib/microvms/<name>/<image>.img

sudo systemctl start microvm@<name>.service

Resizing persist volume to 64GB:

sudo systemctl stop microvm@vm.service

sudo qemu-img resize --format raw /var/lib/microvms/vm/persist.img 64G
sudo e2fsck -f /var/lib/microvms/vm/persist.img
sudo resize2fs /var/lib/microvms/vm/persist.img

sudo systemctl start microvm@vm.service

Clearing a VM

To clear a VM’s state, remove the appropriate image files in /var/lib/microvms/<name>/.

  • persist.img: $HOME and user data
  • nix-store.img: the writable store overlay
  • docker.img: the docker volume (only if mariner.docker.enable is set)
  • waydroid.img: the waydroid volume (only if mariner.waydroid.enable is set)
sudo systemctl stop microvm@<name>.service
sudo rm /var/lib/microvms/<name>/*.img