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

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Date:	Wed, 18 Jul 2012 10:31:28 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Asias He <asias@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	"Michael S. Tsirkin" <mst@...hat.com>, kvm@...r.kernel.org,
	virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH RESEND 5/5] vhost-blk: Add vhost-blk support

Asias He <asias@...hat.com> writes:

> On 07/18/2012 03:10 AM, Jeff Moyer wrote:
>> Asias He <asias@...hat.com> 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
get_user_pages_fast.

Cheers,
Jeff
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