[dm-devel] Re: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems, and dm/md.

[Jens Axboe]

On Wed, May 30 2007, Phillip Susi wrote:
> >That would be the exactly how I understand Documentation/block/barrier.txt:
> >
> >"In other words, I/O barrier requests have the following two properties.
> >1. Request ordering
> >...
> >2. Forced flushing to physical medium"
> >
> >"So, I/O barriers need to guarantee that requests actually get written
> >to non-volatile medium in order." 
> 
> I think you misinterpret this, and it probably could be worded a bit 
> better.  The barrier request is about constraining the order.  The 
> forced flushing is one means to implement that constraint.  The other 
> alternative mentioned there is to use ordered tags.  The key part there 
> is "requests actually get written to non-volatile medium _in order_", 
> not "before the request completes", which would be synchronous IO.

No Stephan is right, the barrier is both an ordering and integrity
constraint. If a driver completes a barrier request before that request
and previously submitted requests are on STABLE storage, then it
violates that principle. Look at the code and the various ordering
options.

-- 
Jens Axboe