From bc450a5189e4e380a626dd8b1ebdee2497a2d7df Mon Sep 17 00:00:00 2001
From: Silvio Rhatto <rhatto@riseup.net>
Date: Tue, 23 Oct 2018 20:32:58 -0300
Subject: Adds support for backing_file

---
 kvmx     | 21 ++++++++++++++++-----
 kvmxfile |  4 ++++
 2 files changed, 20 insertions(+), 5 deletions(-)

diff --git a/kvmx b/kvmx
index 43ea2f2..a1d2012 100755
--- a/kvmx
+++ b/kvmx
@@ -365,12 +365,23 @@ function kvmx_up {
           exit 1
         fi
 
-        echo "Copying base image $baseimage to $image..."
-        if which rsync &> /dev/null; then
-          rsync -ah --sparse --progress $baseimage $image
+        if [ ! -z "$backing_file" ]; then
+          if [ -e "$backing_file" ]; then
+            echo "Creating image $image as an overlay of $backing_file..."
+            backing_file_format="`qemu-img info $backing_file | grep "^file format: " | cut -d : -f 2 | sed -e 's/ //g'` "
+            qemu-img create -o backing_file=$backing_file,backing_fmt=$backing_file_format -f $format $image
+          else
+            echo "Backing file not found: $backing_file"
+            exit 1
+          fi
         else
-          # See https://rwmj.wordpress.com/2010/10/19/tip-making-a-disk-image-sparse/
-          cp --sparse=always $baseimage $image
+          echo "Copying base image $baseimage to $image..."
+          if which rsync &> /dev/null; then
+            rsync -ah --sparse --progress $baseimage $image
+          else
+            # See https://rwmj.wordpress.com/2010/10/19/tip-making-a-disk-image-sparse/
+            cp --sparse=always $baseimage $image
+          fi
         fi
 
         if [ -e "$basekey" ]; then
diff --git a/kvmxfile b/kvmxfile
index 115d150..5f09daf 100644
--- a/kvmxfile
+++ b/kvmxfile
@@ -119,6 +119,10 @@ xrandr="0"
 # Where the guest image is stored
 #image="$HOME/.local/share/kvmx/$VM/box.img"
 
+# Backing file for overlay images
+# See https://wiki.archlinux.org/index.php/QEMU#Overlay_storage_images
+#backing_file="$HOME/.local/share/kvmx/$basebox/box.img"
+
 # Where datafiles are stored: just set this if you know what you're doing
 #datadir="$HOME/.local/share/kvmx/$VM"
 
-- 
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