[Adding NBD mailing list to CC] On Wed, Jul 11, 2018 at 08:43:24AM +0200, Daniel Stenberg wrote: > On Tue, 10 Jul 2018, Richard W.M. Jones wrote: > > >I was wondering if you have any documentation, or general tips, > >for writing a new handler? I was thinking of implementing an NBD > >(Network Block Device) client. > > I'm afraid we don't have much docs for that! :-( I suppose mostly > because it doesn't happen very often. > > I think the best way to do it is to take a look at how other > handlers are written, like perhaps the smb support would be similar > in style and spirit?
After sending this I worked it out by looking at previous commits of backends and the code (for SMTP and others). My ongoing difficulty is understanding the non-blocking model. > Does NBD actually have a URL syntax defined? Not officially. Whatever URL syntax defined needs to be able to specify: - hostname + optional port - OR Unix domain socket - export name In libguestfs we defined our own URL syntax: http://libguestfs.org/guestfish.1.html#a-nbd:-example.com-:port but it's not completely satisfactory since the export name is a free text string and does not necessarily begin with a '/' character. > What's the use case for an application to use this? This is a very good question. My need is for a C client library for accessing NBD servers. I don't believe that one exists. There is plenty of C code for accessing NBD servers (eg. in qemu) but it's encapsulated in other programs and not easy to separate. My idea was to use libcurl instead of writing a new library. The advantage is that curl gives us ‘for free’ a command line tool, multiple language bindings, and widespread availability. This is all relatively uncontroversial I hope. However the complicated bit is that we will eventually need to do more than just simple read/write of blocks of data. The NBD protocol supports other features such as: - discard blocks - efficiently write zeroes - negotiate block size - list exports - query sparseness - TLS with certificates or PSK At some point we'd want to support as many of those features as possible. Curl supports non-orthogonal features for other protocols (eg. FTP active port), so I suppose it's not impossible to extend Curl for NBD, but you may feel that you don't want to do that. Anyway I think even a basic read/write backend is a worthwhile addition at the moment. > Setting up and writing tests for it can probably be a challange. Can we point the tests to something like nbdkit? Rich.  https://sourceforge.net/p/nbd/code/ci/master/tree/doc/proto.md -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-p2v converts physical machines to virtual machines. Boot with a live CD or over the network (PXE) and turn machines into KVM guests. http://libguestfs.org/virt-v2v ------------------------------------------------------------------- Unsubscribe: https://cool.haxx.se/list/listinfo/curl-library Etiquette: https://curl.haxx.se/mail/etiquette.html