Re: [PATCH RESEND 5/5] vhost-blk: Add vhost-blk support

From: Asias He
Date: Wed Jul 18 2012 - 10:43:58 EST

On 07/18/2012 10:31 PM, Jeff Moyer wrote:
Asias He <asias@xxxxxxxxxx> writes:

On 07/18/2012 03:10 AM, Jeff Moyer wrote:
Asias He <asias@xxxxxxxxxx> writes:

vhost-blk is a in kernel virito-blk device accelerator.

This patch is based on Liu Yuan's implementation with various
improvements and bug fixes. Notably, this patch makes guest notify and
host completion processing in parallel which gives about 60% performance
improvement compared to Liu Yuan's implementation.

So, first off, some basic questions. Is it correct to assume that you
tested this with buffered I/O (files opened *without* O_DIRECT)?
I'm pretty sure that if you used O_DIRECT, you'd run into problems (which
are solved by the patch set posted by Shaggy, based on Zach Brown's work
of many moons ago). Note that, with buffered I/O, the submission path
is NOT asynchronous. So, any speedups you've reported are extremely
suspect. ;-)

I always used O_DIRECT to test this patchset. And I mostly used raw
block device as guest image. Is this the reason why I did not hit the
problem you mentioned. Btw, I do have run this patchset on image based
file. I still do not see problems like IO hangs.

Hmm, so do the iovec's passed in point to buffers in userspace? I
thought they were kernel buffers, which would have blown up in

Yes. The iovec's passed in point to userspace buffers. ;-)


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