Re: [Qemu-devel] [RFC] introduce a dynamic library to expose qemu block API

[Paolo Bonzini]

> Whether to provide sync and/or async access is a key question.

Indeed.

> Synchronous APIs are great for writing dedicated tools like dd, cp,
> convert, etc.
> 
> Asynchronous APIs are essential for integrating image file I/O into
> event-driven programs like libvirt.  Here, the ability to do other
> things while image file I/O is in progress is a requirement.  It may
> also be necessary to cancel or timeout if an operation is not making
> progress or the user decides to stop it.
> 
> I think we need to provide both sync and async.  Libraries like
> libssh2 and libcurl already do this so their APIs can be used as a
> starting point for async I/O.

If we want to provide an asynchronous API, the easiest thing would
be to provide a GSource and that's it.  That would even make sense for
QEMU itself, in fact.

What I'm worried about, is how to support _both_ synchronous and
asynchronous access.  I'd like the library to be clean of things like
qemu_aio_wait() and qemu_aio_flush(), at least in the beginning.
That's why I think async can come later, once we actually get
applications needing it.  Right now, libvirt's requirements are
simple (e.g. probing the backing file chain) and would be synchronous
anyway.

Paolo