I've added support for filters to the nbdkit upstream repository (version >= 1.1.27). Filters allow you to modify plugins. A simple example would be selecting a partition from a plugin: nbdkit --filter=partition file file=disk.img partition=2 ^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^ which would serve the second partition only from disk.img. For more information see the nbdkit(1) and nbdkit-filter(3) man pages. For nbdkit plugins written in C we offer a long term stable ABI, where you can write a plugin (in C) now and have it work on all future versions of nbdkit. This means plugins can be distributed separately from nbdkit. However for nbdkit filters we haven't decided what, if any, ABI guarantees we will offer. The reason for this is simple: filters work by filtering plugin methods, but if we add new plugin methods then existing filters won't "see" the new methods. In some common cases this could even cause data corruption, for example if a filter is modifying file offsets like the partition filter above, then the new methods would write to the wrong part of the file. So the TL;DR is, for now, if you write a filter it is far better to submit it upstream so that we can keep it up to date. Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-df lists disk usage of guests without needing to install any software inside the virtual machine. Supports Linux and Windows. http://people.redhat.com/~rjones/virt-df/