Re: [PATCH] ide write barrier support

[Nick Piggin <piggin@cyberone.com.au>]

Daniel Phillips wrote:

>On Thursday 23 October 2003 18:23, Jens Axboe wrote:
>
>>On Thu, Oct 23 2003, Daniel Phillips wrote:
>>
>>>I'm specifically interested in working out the issues related to stacked
>>>virtual devices, and there are many.  Let me start with an easy one.
>>>
>>>Consider a multipath virtual device that is doing load balancing and
>>>wants to handle write barriers efficiently, not just allow the
>>>downstream queues to drain before allowing new writes.  This device
>>>wants to send a write barrier to each of the downstream devices,
>>>however, we have only one write request to carry the barrier bit.  How
>>>do you recommend handling this situation?
>>>
>>That needs something to hold the state in, and a bio per device. As
>>they complete, mark them as such. When they all have completed, barrier
>>is done.
>>
>>That's just an idea, I'm sure there are other ways. Depending on how
>>complex it gets, it might not be a bad idea to just let the queues drain
>>though. I think I'd prefer that approach.
>>
>
>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.
>
>Assuming this is worth doing, how do we issue N barriers to the downstream 
>devices when we have only one incoming barrier write?
>

You would do this in the multipath code, wouldn't you?

Anyway, I might be missing something, but I don't think draining the
queue will guarantee that writeback caches will go to permanent storage.