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

From: Jeff Moyer
Date: Wed Jul 18 2012 - 10:31:27 EST

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

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