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Date: Wed, 10 Dec 2008 20:08:10 +0100
From: Andrea Arcangeli <aarcange@redhat.com>
Subject: Re: [Qemu-devel] [RFC] Replace posix-aio with custom thread pool
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To: Anthony Liguori <anthony@codemonkey.ws>
Cc: Gerd Hoffmann <kraxel@redhat.com>, kvm-devel <kvm@vger.kernel.org>, qemu-devel@nongnu.org

On Wed, Dec 10, 2008 at 12:50:17PM -0600, Anthony Liguori wrote:
> But opening twice means that you lose coherency with NFS.

Not sure why. They're not running from different nfs clients. If this
really isn't feasible, other ways to go would be to stick with a
single thread and add kernel aio to fix seeking I/O bandwidth (so cfq
indexing the bucket by the thread id will also work ok).

Alternatively we can support readv/writev only with kernel aio, no
threads, and we emulate it with a flood of pread/pwrite on the other
host os plus we set DEBUG_BOUNCE where readv/writev is emulated. But
then linux will be the only host OS benefiting from zerocopy.

If we never want to add kernel aio, we can also add preadv/pwritev to
linux but it remains a linux-only solution, so going with kernel aio
for a linux-only solution is surely better than the workaround with
userland threads plus preadv/pwritev. Still Al or somebody should add
preadv/pwritev, those are needed and useful as shown here.