[Qemu-devel] Re: [PATCH] virtio-spec: document block CMD and FLUSH

[Avi Kivity]

On 05/04/2010 07:38 AM, Rusty Russell wrote:
> On Fri, 19 Feb 2010 08:52:20 am Michael S. Tsirkin wrote:
>    
> > I took a stub at documenting CMD and FLUSH request types in virtio
> > block.  Christoph, could you look over this please?
> >
> > I note that the interface seems full of warts to me,
> > this might be a first step to cleaning them.
> >      
> ISTR Christoph had withdrawn some patches in this area, and was waiting
> for him to resubmit?
>
> I've given up on figuring out the block device.  What seem to me to be sane
> semantics along the lines of memory barriers are foreign to disk people: they
> want (and depend on) flushing everywhere.
>
> For example, tdb transactions do not require a flush, they only require what
> I would call a barrier: that prior data be written out before any future data.
> Surely that would be more efficient in general than a flush!  In fact, TDB
> wants only writes to *that file* (and metadata) written out first; it has no
> ordering issues with other I/O on the same device.
>    

I think that's SCSI ordered tags.

> A generic I/O interface would allow you to specify "this request depends on 
> these
> outstanding requests" and leave it at that.  It might have some sync flush
> command for dumb applications and OSes.  The userspace API might be not be as
> precise and only allow such a barrier against all prior writes on this fd.
>    

Depends on all previous requests, and will commit before all following 
requests.  ie a full barrier.

> ISTR someone mentioning a desire for such an API years ago, so CC'ing the
> usual I/O suspects...
>    

I'd love to see TCQ exposed to user space.

--
error compiling committee.c: too many arguments to function