Optimize VM image setup

These changes save over 5s on a VM setup phase of a typical
armv7l build. Instead of asking mkfs.ext4 to create a journal
(which is causing it to do flushed writes of lots of zero blocks)
and immediately deleting it afterwards, simply don't create it
initially. This alone saves 2s.

Prefering fallocate to make use of persistent preallocation
avoids fragmentation and small other fixes further speed up
the initial build setup phase by several seconds typically.

build

@@ -28,8 +28,10 @@ repos=()
 old_packages=()
 
 # defaults for vm_img_mkfs
-vm_img_mkfs_ext4='mkfs.ext4 -m 0 -q -F'
-vm_img_tunefs_ext4='tune2fs -c 0 -O ^has_journal'
+vm_img_mkfs_ext4_options='-O ^has_journal,^huge_file,^resize_inode,sparse_super'
+vm_img_mkfs_ext4_extra='-E lazy_itable_init,discard'
+vm_img_mkfs_ext4="mkfs.ext4 -m 0 -q -F $vm_img_mkfs_ext4_options $vm_img_mkfs_ext4_extra"
+vm_img_tunefs_ext4='tune2fs -c 0'
 vm_img_mkfs_ext3='mkfs.ext3 -m 0 -q -F'
 vm_img_tunefs_ext3='tune2fs -c 0 -o journal_data_writeback'
 vm_img_mkfs_ext2='mkfs.ext2 -m 0 -q -F'
@@ -621,6 +623,26 @@ copy_oldpackages()
     done
 }
 
+vm_img_create()
+{
+    local img="$1"
+    local size="$2"
+
+    echo "Creating $img (${size}M)"
+    mkdir -p "${img%/*}" || cleanup_and_exit 3
+
+    # prefer fallocate, which avoids fragmentation
+    r=1
+    if type -p fallocate > /dev/null; then
+        fallocate -l "${size}M" "$img"
+        r=$?
+    fi
+    # fall back to dd method if fallocate is not supported
+    if [ "$r" -gt "0" ]; then
+        dd if=/dev/zero of="$img" bs=1M count=0 seek="$size" || cleanup_and_exit 3
+    fi
+}
+
 vm_img_mkfs()
 {
     local fs="$1"
@@ -640,7 +662,6 @@ vm_img_mkfs()
     if test -n "$tunefs" ; then
 	$tunefs "$img" || cleanup_and_exit 3
     fi
-
 }
 
 background_monitor_process()
@@ -671,7 +692,7 @@ background_monitor_process()
 
     # disk storage usage
     if type -p df >& /dev/null; then
-      c=(`df -m / | tail -n 1`)
+      c=(`df -m / 2>/dev/null | tail -n 1`)
 
       if [ ${c[2]} -gt $max_disk ]; then
         max_disk="${c[2]}"
@@ -1561,18 +1582,14 @@ if test -z "$RUNNING_IN_VM" ; then
 	    fi
 	fi
 	if test ! -e "$VM_IMAGE"; then
-	    echo "Creating $VM_IMAGE (${VMDISK_ROOTSIZE}M)"
-	    mkdir -p "${VM_IMAGE%/*}"
-	    dd if=/dev/zero of="$VM_IMAGE" bs=1M count=0 seek="$VMDISK_ROOTSIZE" || cleanup_and_exit 3
+	    vm_img_create "$VM_IMAGE" "$VMDISK_ROOTSIZE"
+
 	    if test -z "$CLEAN_BUILD" ; then
-		vm_img_mkfs "$VMDISK_FILESYSTEM" "$VM_IMAGE" || cleanup_and_exit 3
+		vm_img_mkfs "$VMDISK_FILESYSTEM" "$VM_IMAGE"
 	    fi
 	fi
 	if test -n "$VM_SWAP" -a ! -e "$VM_SWAP" -a ! -b "$VM_SWAP"; then
-	    # setup VM_SWAP
-	    echo "Creating $VM_SWAP (${VMDISK_SWAPSIZE}M)"
-	    mkdir -p "${VM_SWAP%/*}"
-	    dd if=/dev/zero of="$VM_SWAP" bs=1M count=0 seek="$VMDISK_SWAPSIZE" || cleanup_and_exit 3
+	    vm_img_create "$VM_SWAP" "$VMDISK_SWAPSIZE"
 	fi
 	if test ! -e "$VM_IMAGE" ; then
 	    echo "you need to create $VM_IMAGE first"