[Libguestfs] Few libnbd questions/concerns

[Richard W.M. Jones]

On Tue, Jun 25, 2019 at 02:58:34PM +0200, Martin Kletzander wrote:
> Here are few things I found out when using libnbd that might be perfectly fine
> or maybe just an oversight, but I wanted to point them out.  It's nothing major.
> 
> When running a program with `nbdkit -U - --run ...`, the $nbd parameter gets
> expanded to nbd:unix:/path/to/socket.  When this string is passed to
> nbd_connect_uri(), it does not return an error (even though it is not a valid
> URL), but what's more it treats it as "nbd://localhost", which might be a completely different server (that actually happened to me and it was kind of confusing).

There's a bit of magic in nbdkit about $nbd.  It also predates our
standardization effort for NBD URLs which is going on upstream.

That needs to be fixed, but in the meantime just use $unixsocket
instead (see nbdkit-captive(1) man page for the full details).

> When nbd_block_status() callback returns -1, the nbd_block_status() function
> reports:
> 
>  command failed: Protocol error (errno = 71)
> 
> which is a bit confusing to me.  I could be nicer to have that report that it
> really was the callback that caused this.

Not sure about this one - Eric?

> One last thing is that I could not find any definition for the flags
> of "base:allocation" metadata (NBD_STATE_HOLE and NBD_STATE_ZERO).
> It might just be things that are still missing due to an early stage
> of libnbd, or it might not belong there or it might be me misusing
> something.

Have a look at the NBD protocol doc:

https://github.com/NetworkBlockDevice/nbd/blob/master/doc/proto.md

Rich.

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