Add disk preallocation support

preallocation=metadata is now the default as it offers the best size/performance.

README.md

@@ -271,6 +271,20 @@ Add additional lines to your virtual machine configuration:
   * `cpu_cores="4"` - Specify the number of CPU cores allocated to the VM
   * `ram="4G"` - Specify the amount of RAM to allocate to the VM
   * `disk="16G"` - Specify the size of the virtual disk allocated to the VM
+
+
+## Disk preallocation
+
+Preallocation mode (allowed values: `off`, `metadata` (default), `falloc`, `full`).
+An image with preallocated metadata is initially larger but can improve performance
+when the image needs to grow. `falloc` and `full` preallocations are like the
+same options of raw format, but sets up metadata also.
+
+Specify what disk preallocation should be used, if any, when creating the system
+disk image by adding a line like this to your VM configuration.
+
+  * `preallocation="metadata"`
+
 ## Floppy disks
 
 If you're like [Alan Pope](https://popey.com) you'll probably want to mount a

quickemu

@@ -235,10 +235,19 @@ function vm_boot() {
   if [ ! -f "${disk_img}" ]; then
       # If there is no disk image, create a new image.
       mkdir -p "${VMDIR}" 2>/dev/null
-      if ! ${QEMU_IMG} create -q -f qcow2 "${disk_img}" "${disk}"; then
+      case ${preallocation} in
+        off|metadata|falloc|full) true;;
+        *)
+          echo "ERROR! ${preallocation} is an unsupported disk preallocation option."
+          exit 1;;
+      esac
+
+      # https://blog.programster.org/qcow2-performance
+      if ! ${QEMU_IMG} create -q -f qcow2 -o lazy_refcounts=on,preallocation="${preallocation}" "${disk_img}" "${disk}"; then
         echo "ERROR! Failed to create ${disk_img}"
         exit 1
       fi
+
       if [ -z "${iso}" ] && [ -z "${img}" ]; then
         echo "ERROR! You haven't specified a .iso or .img image to boot from."
         exit 1
@@ -618,6 +627,7 @@ guest_os="linux"
 img=""
 iso=""
 port_forwards=()
+preallocation="metadata"
 ram=""
 usb_devices=()
 virtio_blk="on"