To-do list for nbdkit
======================================================================

General ideas for improvements
------------------------------

* Listen on specific interfaces or protocols (eg. only IPv6).

* Performance - measure and improve it.

* Exit on last connection (the default behaviour of qemu-nbd unless
  you use -t).

* Limit number of incoming connections (like qemu-nbd -e).

* For parallel plugins, only create threads on demand from parallel
  client requests, rather than pre-creating all threads at connection
  time, up to the thread pool size limit.  Of course, once created, a
  thread is reused as possible until the connection closes.

* Async callbacks.  The current parallel support requires one thread
  per pending message; a solution with fewer threads would split
  low-level code between request and response, where the callback has
  to inform nbdkit when the response is ready:
  https://www.redhat.com/archives/libguestfs/2018-January/msg00149.html

* More NBD protocol features. Qemu has implemented Structured Replies,
  which allows for more efficient serving of sparse files.  Also in
  the upstream pipeline: proposals for block status and online resize.

* Add a callback to let plugins request minimum alignment for the
  buffer to pread/pwrite; useful for a plugin utilizing O_DIRECT or
  other situation where pre-aligned buffers are more efficient.
  Ideally, a blocksize filter would honor strict alignment below and
  advertise loose alignment above; all other filters (particularly
  ones like offset) can fail to initialize if they can't guarantee
  strict alignment and don't want to deal with bounce buffers.

* Add per-connection caching of .can_FOO callbacks (we already have
  some: .can_write is only called once, but .can_fua is called on
  every request with the FUA flag set).

* Test that zero-length read/write requests behave correctly
  (NBD protocol says they are unspecified).

* Test and document how to run nbdkit from inetd and xinetd in
  nbdkit-service(1).

Suggestions for plugins
-----------------------

Note: qemu supports other formats such as libssh, libnfs, iscsi,
gluster and ceph/rbd, and while similar plugins could be written for
nbdkit there is no compelling reason unless the result is better than
qemu-nbd.  For the majority of users it would be better if they were
directed to qemu-nbd for these use cases.

* XVA files

  https://lists.gnu.org/archive/html/qemu-devel/2017-11/msg02971.html
  is a partial solution but it needs cleaning up.

* Create ext2 filesystems

  Similar to nbdkit-floppy-plugin which creates FAT32 filesystems, we
  could also create ext2 filesystems.  This is relatively
  straightforward using libext2fs (part of e2fsprogs).  In fact we
  already use this library for the unrelated nbdkit-ext2-plugin.

nbdkit-nbd-plugin could use enhancements:

* FUA passthrough, rather than extra FLUSH calls.  For this, .can_fua
  must be exposed to plugins (not just filters), and needs to be
  tri-state rather than just bool.

* Enable client-side TLS (right now, the nbd plugin allows us to
  support an encrypted client connecting to a plain server; but we
  would need TLS to support a plain client connecting to an encrypted
  server).

* Support for connecting to a server over IP rather than just Unix
  sockets.

nbdkit-floppy-plugin needs boot sector support.  In theory this is
easy (eg. using SYSLINUX), but the practical reality of making a fully
bootable floppy is rather more complex.

Suggestions for filters
-----------------------

* tar plugin should really be a filter

* xz plugin should really be a filter

* gzip plugin should really be a filter

* masking plugin features for testing clients (see 'nozero' and 'fua'
  filters for examples)

nbdkit-cache-filter needs considerable work:

* allow the user to limit the total size of the cache

* handle ENOSPC errors

* implement some cache replacement policies

* some sort of background task or thread to write out dirty blocks

Composing nbdkit
----------------

Filters allow certain types of composition, but others would not be
possible, for example RAIDing over multiple nbd sources.  Because the
plugin API limits us to loading a single plugin to the server, the
best way to do this (and the most robust) is to compose multiple
nbdkit processes.

The nbd plugin (plugins/nbd) already contains an NBD client, so we
could factor this client out and make it available to other plugins to
use.
