diff --git a/src/disk.sh b/src/disk.sh
index 9a835d7..165f5b2 100644
--- a/src/disk.sh
+++ b/src/disk.sh
@@ -433,6 +433,14 @@ fi
 
 DISK_EXT="$(fmt2ext "$DISK_FMT")" || exit $?
 
+if [ -z "$ALLOCATE" ]; then
+  if [[ "${DISK_FMT,,}" == "raw" ]]; then
+    ALLOCATE="Y"
+  else
+    ALLOCATE="N"
+  fi
+fi
+
 if [[ "$ALLOCATE" == [Nn]* ]]; then
   DISK_TYPE="growable"
   DISK_ALLOC="preallocation=off"
diff --git a/src/reset.sh b/src/reset.sh
index 6690538..8b1cb5d 100644
--- a/src/reset.sh
+++ b/src/reset.sh
@@ -12,8 +12,8 @@ trap 'error "Status $? while: $BASH_COMMAND (line $LINENO/$BASH_LINENO)"' ERR
 # Docker environment variables
 
 : ${BOOT:=''}           # URL of the ISO file
-: ${DEBUG:='N'}         # Disable debugging mode
-: ${ALLOCATE:='N'}      # Disable preallocation
+: ${DEBUG:='N'}         # Disable debugging
+: ${ALLOCATE:=''}       # Preallocate diskspace
 : ${ARGUMENTS:=''}      # Extra QEMU parameters
 : ${CPU_CORES:='1'}     # Amount of CPU cores
 : ${RAM_SIZE:='1G'}     # Maximum RAM amount