Quick Introduction to NBD
[About 2 mins 30 seconds]

* Heading: Linux 2.1.55 (1997)
- Diagram showing two machines
- Physical disk attached to one
- Exported to /dev/nbd0 on the other
- NBD_CMD_READ (0) or NBD_CMD_WRITE (1) only

Network Block Device is a protocol for exporting a disk from
one machine to another over a network.  It was added to
Linux 2.1.55 in 1997 and started off very simple with just two commands.

* Heading: Linux 5 (2019)
- Same diagram annotated with additional commands:
- DISC, FLUSH, TRIM, CACHE, WRITE_ZEROES, BLOCK_STATUS, RESIZE

In 2019 NBD is quite a bit more complicated, but still
overall a simple protocol.

* Heading: Exporting a point in time snapshot over NBD
- Diagram showing qemu exporting a point in time snapshot of a live guest
- blockdev-backup device=A target=Target sync=none job-id=job1
- nbd-server-start addr={"type":"unix","data":{"path":"./nbd-sock"}}
- nbd-server-add device=Target

Our use of NBD today is quite a bit different from how it was
intended back in 1997.  Here are some ways we're using it today.

As a way to export a point in time snapshot from qemu.

* Heading: Virt-v2v using nbdkit to read a VDDK disk
- Diagram showing virt-v2v + nbdkit + vddk + VMware

As a way to read a VDDK disk and convert it to run on KVM.

* Heading: qemu-nbd as a high performance server
- Diagram showing qemu-nbd used by RHV ImageIO

As a high performance server, serving qcow2 files for ImageIO.

* Heading: qemu-img convert from RHV ImageIO to KubeVirt
- Diagram showing nbdkit -> qemu-img convert -> nbdkit

Copying disk images efficiently between KVM-based management systems.

* Heading: nbdkit linuxdisk used to boot remote machines
- Diagram showing linuxdisk synthesizing an initramfs/filesystem

Using it as a replacement for TFTP when PXE-booting machines
over the network.

* Heading: Making disk images for testing
- Diagram showing an nbdkit shell script plugin

As a general purpose tool to make disk images for testing.

You can think of NBD as a universal protocol that you can use
whenever you need to access a virtual machine disk, disk image,
or block device between two processes, whether or not the
processes are running on the same machine.