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Re: [Xen-devel] [PATCH] Blktap: Userspace file-based image support. (RFC)

  • To: "Rusty Russell" <rusty@xxxxxxxxxxxxxxx>
  • From: "Andrew Warfield" <andrew.warfield@xxxxxxxxxxxx>
  • Date: Wed, 28 Jun 2006 22:24:28 -0700
  • Cc: Xen Developers <xen-devel@xxxxxxxxxxxxxxxxxxx>, Julian Chesterfield <julian.chesterfield@xxxxxxxxxxxx>
  • Delivery-date: Wed, 28 Jun 2006 22:24:52 -0700
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  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>

        I like the idea of block servers in userspace, but I'm curious.  When I
wrote the simple share block server I couldn't see an obvious
justification for multiple outstanding requests (with AIO/threads and
all that entails), so I went for the trivial single request approach.
It seems to me that the backend doesn't have much information the front
end doesn't have.

Hi Rusty,

  not sure I see what you are asking.  A very early version of the
code just did  synchronous dispatch (one blocking request at a time)
and was, as you might expect, very slow.  You clearly want to keep the
block request queues as full as possible to amortize seeks... AIO just
lets me issue batches of requests at once, and so minimizes context
switching through userland -- which was something I was worried about
causing overhead on x86_64.  I don't really think it adds that much

  The process-per-disk thing is optional in the current code, you
could just as easily build a single-threaded user backend.  The
current model hopefully buys you a bit of resiliency against crashes
and maps per-disk request streams in a fairly clean way down onto the
block sheduler.

Am I missing your point?


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