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

[Rusty Russell <rusty@rustcorp.com.au>]

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ISTR Christoph had withdrawn some patches in this area, and was waiting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.

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.

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

Cheers,
Rusty.