crosvm supports virtio-block device that works as a disk for the guest.

First, create a ext4 (or whatever file system you want) disk file.

fallocate -l 1G disk.img
mkfs.ext4 disk.img

Then, pass it with --block flag so the disk will be exposed as /dev/vda, /dev/vdb, etc. The device can be mounted with the mount command.

crosvm run \
  --block disk.img
  ... # usual crosvm args

To expose the block device as a read-only disk, you can add the ro flag after the disk image path:

crosvm run \
  --block disk.img,ro
  ... # usual crosvm args


If you use a block device as guest's rootfs, you can add the root flag to the --block parameter:

crosvm run \
  --block disk.img,root
  ... # usual crosvm args

This flag automatically adds a root=/dev/vdX kernel parameter with the corresponding virtio-block device name and read-only (ro) or read-write (rw) option depending on whether the ro flag has also been specified or not.


The --block parameter support additional options to enable features and control disk parameters. These may be specified as extra comma-separated key=value options appended to the required filename option. For example:

crosvm run
  --block disk.img,ro,sparse=false,o_direct=true,block_size=4096,id=MYSERIALNO
  ... # usual crosvm args

The available options are documented in the following sections.


  • Syntax: sparse=(true|false)
  • Default: sparse=true

The sparse option controls whether the disk exposes the thin provisioning discard command. If sparse is set to true, the VIRTIO_BLK_T_DISCARD request will be available, and it will be translated to the appropriate system call on the host disk image file (for example, fallocate(FALLOC_FL_PUNCH_HOLE) for raw disk images on Linux). If sparse is set to false, the disk will be fully allocated at startup (using fallocate() or equivalent on other platforms), and the VIRTIO_BLK_T_DISCARD request will not be supported for this device.


  • Syntax: o_direct=(true|false)
  • Default: o_direct=false

The o_direct option enables the Linux O_DIRECT flag on the underlying disk image, indicating that I/O should be sent directly to the backing storage device rather than using the host page cache. This should only be used with raw disk images, not qcow2 or other formats. The block_size option may need to be adjusted to ensure that I/O is sufficiently aligned for the host block device and filesystem requirements.

Block size

  • Syntax: block_size=BYTES
  • Default: block_size=512

The block_size option overrides the reported block size (also known as sector size) of the virtio-block device. This should be a power of two larger than or equal to 512.


  • Syntax: id=DISK_ID
  • Default: No identifier

The id option provides the virtio-block device with a unique identifier. The DISK_ID string must be 20 or fewer ASCII printable characters. The id may be used by the guest environment to uniquely identify a specific block device rather than making assumptions about block device names.

The Linux virtio-block driver exposes the disk identifer in a sysfs file named serial; an example path looks like /sys/devices/pci0000:00/0000:00:02.0/virtio1/block/vda/serial (the PCI address may differ depending on which other devices are enabled).


The crosvm block device supports run-time resizing. This can be accomplished by starting crosvm with the -s control socket, then using the crosvm disk command to send a resize request:

crosvm disk resize DISK_INDEX NEW_SIZE VM_SOCKET

  • DISK_INDEX: 0-based index of the block device (counting all --block in order).
  • NEW_SIZE: desired size of the disk image in bytes.
  • VM_SOCKET: path to the VM control socket specified when running crosvm (-s/--socket option).

For example:

# Create a 1 GiB disk image
truncate -s 1G disk.img

# Run crosvm with a control socket
crosvm run \
  --block disk.img,sparse=false \
  -s /tmp/crosvm.sock \
  ... # other crosvm args

# In another shell, extend the disk image to 2 GiB.
crosvm disk resize \
  0 \
  $((2 * 1024 * 1024 * 1024)) \

# The guest OS should recognize the updated size and log a message:
#   virtio_blk virtio1: [vda] new size: 4194304 512-byte logical blocks (2.15 GB/2.00 GiB)

The crosvm disk resize command only resizes the block device and its backing disk image. It is the responsibility of the VM socket user to perform any partition table or filesystem resize operations, if required.