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Date: Wed, 18 Jul 2012 06:42:24 -0400 (EDT)
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [RFC] introduce a dynamic library to expose qemu
	block API
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To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: Anthony Liguori <aliguori@us.ibm.com>, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>, Markus Armbruster <armbru@redhat.com>, Michael Tokarev <mjt@tls.msk.ru>, qemu-devel@nongnu.org, =?utf-8?Q?Llu=C3=ADs?= <xscript@gmx.net>, Blue Swirl <blauwirbel@gmail.com>, Stefan Weil <weil@mail.berlios.de>, Hannes Reinecke <hare@suse.de>, Wenchao Xia <xiawenc@linux.vnet.ibm.com>

> 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