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

[Anthony Liguori <aliguori@xxxxxxxxxx>]

Rusty Russell wrote:
> On Mon, 2006-06-19 at 09:19 -0700, Andrew Warfield wrote:
>   
> > Attached to this email is a patch containing the (new and improved)
> > blktap Linux driver and associated userspace tools for Xen.  In
> > addition to being more flavourful, containing half the fat, and
> > removing stains twice as well as the old driver, this stuff adds a
> > userspace block backend and let you use raw (without loopback), qcow,
> > and vmdk-based image files for your domUs.  There's also a fun little
> > driver that provides a shared-memory block device which, in
> > combination with OCFS2, represents a cheap-and-cheerful fast shared
> > filesystem between multiple domUs.
> >     
> Hi Andrew,
>
>         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),
Are you thinking of posix-aio?  posix-aio is "emulated" with threads and 
normal read/select calls.  The performance isn't that great.
I believe blktap is using linux-aio which doesn't use threads (it uses 
the linux specific interface).  I've seen a number of benchmarks where 
linux-aio is significantly better than posix-aio.
Regards,

Anthony Liguori

>  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.
>
> Just wondered if you'd tried a naive approach first...
>
> Thanks!
> Rusty.
>   

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