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Date: Tue, 09 Dec 2008 16:51:57 +0100
From: Gerd Hoffmann <kraxel@redhat.com>
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Subject: Re: [Qemu-devel] [RFC] Replace posix-aio with custom thread pool
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To: qemu-devel@nongnu.org
Cc: Anthony Liguori <aliguori@us.ibm.com>, kvm-devel <kvm@vger.kernel.org>

  Hi,

> 2) it's impossible to add new interfaces and we need a vectored read/write
> operation to properly support a zero-copy API.

I'm eager to try vectored block ops for the xenbus block backend.

> It performs at least as well as the current posix-aio code (in some
> circumstances, even better).

Well, I see a massive slowdown when switching from sync to aio in the
xen backend code.  I think the reason is that due to the lack of a
vectored interface (and thus /me submitting separate aio requests for
each iovec element) stuff gets parallelized *way* too much and disk seek
times are killing me.

> My only concern here is non-Linux Unices like FreeBSD.  They have kernel support
> for posix-aio.  Since we cannot extend those interfaces though, I think that
> even on those platforms we should still use a thread pool.

Which might change some day in the future when we manage to get iovec
support into posix-aio specs.

I think the interface should use qemu-prefixed function and struct
names.  The we can trivially map them to a system-provided aio
implementation without worrying about name clashes.

cheers,
  Gerd