Re: [dm-devel] [PATCH UPDATED 4/5] dm: implement REQ_FLUSH/FUA support for request-based dm

[Mikulas Patocka <mpatocka redhat com>]

On Wed, 1 Sep 2010, Tejun Heo wrote:

> Hello,
> 
> On 09/01/2010 12:31 PM, Mikulas Patocka wrote:
> > My recommended approach to this (on non-request-based dm) is to simply let 
> > the current barrier infrastructure be as it is --- you don't need to 
> > change it now, you can simply map FUA write to barrier write and FLUSH to 
> > zero-data barrier --- and it won't cause any data corruption. It will just 
> > force unneeded I/O queue draining.
> > 
> > Once FLUSH+FUA interface is finalized and committed upstream, we can 
> > remove that I/O queue draining from dm to improve performance.
> 
> Unfortunately, it doesn't work that way.  The current dm
> implementation depends on block layer holding the queue while a
> barrier sequence is in progress which the new implementation doesn't
> do anymore (the whole point of this conversion BTW).

That may be true for request-based dm (I don't know).

But bio-based dm doesn't depend on it, I wrote it and I didn't rely on 
that.

Mikulas

> Thanks.
> 
> -- 
> tejun
>