Re: [PATCH] ide write barrier support

[Helge Hafting <helgehaf@aitel.hist.no>]

On Thu, Oct 23, 2003 at 07:20:39PM +0200, Daniel Phillips wrote:
> These are essentially the same, they both rely on draining the downstream 
> queues.  But if we could keep the downstream queues full, bus transfers for 
> post-barrier writes will overlap the media transfers for pre-barrier writes, 
> which would seem to be worth some extra effort.
> 
> To keep the downstream queues full, we must submit write barriers to all the 
> downstream devices and not wait for completion.  That is, as soon as a 
> barrier is issued to a given downstream device we can start passing through 
> post-barrier writes to it.
> 
This approach may fail:

a. Some pre-barrier writes go to all devices
b. barrier is sent to all devices
c. Post-barrier writes go to all devices
d. drive 1 commits all its pre-barrier writes, then
   commits its post-barrier writes.
e. drive 2 is slow and havent done the pre-barrier writes yet
f. power is lost - leaving inconsistent devices.

The problem is that drive 1 don't know wether drive 2
did the barrier yet.

Helge Hafting