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

["Andrew Warfield" <andrew.warfield@xxxxxxxxxxxx>]

Hi Anthony,


> What is img-sp?  Is this blktap + a physical device or is this blktap
> with something like qcow?
Oops, good question.  This is blktap backed off of a sparse image file
(generated with something along the lines of "dd if=/dev/zero
of=./scratch.img bs=1024 seek=<big number>").  So this test is
directly comparable to blkback with a loopback-mounted image, which is
what's shown on the next line.

> The numbers a tad worse than I'd expect them to be if it was a physical
> device.  Theoretically, linux-aio is inserting requests directly into
> the backend.  I expect there to be a certain amount of CPU overhead from
> context switching but since it's still zero-copy, I wouldn't expect less
> CPU usage and less throughput.
>
> Any idea why this is or am I just totally misunderstanding how things
> should behave :-)
Performance on raw devices is certainly better than on images -- I
didn't have a spare partition to work with on my test box this morning
(maybe I can use that as an excuse to get an extra disk), but will get
some results posted on this asap.

> A very much like the idea of a userspace block device backend.  Have you
> considered what it would take to completely replace blkback with a
> userspace backend?  I'm also curious why you choose a character device
> to interact with the ring queue instead of just attaching to the ring
> queue directly in userspace.
The current blktap code has functional parity with blkback.  Just
change 'phys:' to 'tap:aio:' in your vm config files and you're set.

> I think the whole discussion of COW support is orthogonal to a userspace
> backend FWIW so I'll save that part of the discussion for another thread :-)
Fair enough, I'll look forward to it. ;)

a.

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