10_USEFUL_TOOLS.md

QEMU Disk utils/LibGuestFS handy commands

1. Create a sparse (dynamically allocated) disk:

   # Yields a 100 GiB (not GB) disk.
   # `-f qcow2` is required, as the default is raw (which preallocates all the space).
   qemu-img create -f qcow2 "$DISK_IMAGE" 100G

2. Create a diff disk:

   qemu-img create -f qcow2 -b "$DISK_IMAGE" "$DISK_IMAGE.diff"

3. Create a compressed copy of a disk (can also be used to merge a diff image):

   # [-p]rogress, [-O]utput format, [-c]ompressed
   qemu-img convert -p -O qcow2 -c "$DISK_IMAGE" "$DISK_IMAGE.merged"

4. Create a compressed copy without unallocated space (use --tmp, otherwise /tmp is used!!):

   virt-sparsify --compress --tmp $(dirname "$DISK_IMAGE") "$DISK_IMAGE" "$DISK_IMAGE.sparse.compressed"

5. Import a directory in a disk:

   virt-copy-in -a "$DISK_IMAGE" /tmp/pizza /

6. Mount a disk using qemu-nbd:

   PARTITION_NUMBER=4                           # 1-based
   modprobe nbd max_part=8
   qemu-nbd --connect=/dev/nbd0 "$DISK_IMAGE"   # wait a second after this
   partprobe /dev/nbd0
   mount "/dev/nbd0p"$PARTITION_NUMBER" /mnt

7. and unmount:

   umount /mnt
   qemu-nbd --disconnect /dev/nbd0
   rmmod nbd

8. Mounting a disk can also be used to shrink an image:

   # mount the disk (5.)
   dd -status=progress if=/dev/zero of=/mnt/ZEROFILE
   rm /mnt/ZEROFILE
   # now umount the disk (6.)
   # now compress the image (2.)

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