Re: Help with implementing some form of barriers in 3.0 kernels.

[Vivek Goyal <vgoyal@redhat.com>]

On Wed, Sep 07, 2011 at 04:16:20PM -0400, Christoph Hellwig wrote:
> [Hmm, for some reason I never manage to receive Konrads mails directly,
>  but only get the replies, or copies via the list]
> 
> On Wed, Sep 07, 2011 at 02:17:40PM -0400, Vivek Goyal wrote:
> > On Wed, Sep 07, 2011 at 01:48:32PM -0400, Konrad Rzeszutek Wilk wrote:
> > > Hey Christoph,
> > > 
> > > I was wondering what you think is the proper way of implementing a
> > > backend to support the 'barrier' type requests? We have this issue were
> > > there are 2.6.36 type guests that still use barriers and we would like
> > > to support them properly. But in 3.0 there are no barriers - hence
> > > the question whether WRITE_fLUSH_FUA would be equal to WRITE_BARRIER?
> > 
> > I think WRITE_FLUSH_FUA is not same as WRITE_BARRIER. Because it does
> > not ensure request ordering. A request rq2 which is issued after rq1 (with
> > WRITE_flush_FUA), can still finish before rq1. In the past WRITE_BARRIER
> > would not allow that.
> > 
> > So AFAIK, WRITE_flush_fua is not WRITE_BARRIER.
> 
> Indeed.  And while most guests won't care some will.  E.g. reiserfs
> which is the standard filesystem in most SuSE guests, which happen to
> be fairly popular with Xen.
> 
> I'd suggest you look at the pre-2.6.36 barrier implementation and see
> if you can move that into xen-blkfront.
> 
> For the qemu side doing this is a bit easier as you'll just have to wait
> for all pending aio requests to complete.  The current qemu xen disk
> code gets thus horribly wrong, though.

I have a basic question. In old guest why BARRIER handling on request
queue is not sufficient for sequencing and ordering of requests and why
xen-blkfront and qemu have to do something about it.

I am also wondering if virtio-blk have similar issues?

Thanks
Vivek