Re: Linux 2.6.29

[Ric Wheeler]

Linus Torvalds wrote:
> On Wed, 25 Mar 2009, Ric Wheeler wrote:
>   
> > One concern with doing this above the file system is that you are not in the
> > context of a transaction so you have no clean promises about what is on disk
> > and persistent when. Flushing the cache is primitive at best, but the way
> > barriers work today is designed to give the transactions some pretty critical
> > ordering semantics for journalling file systems at least.
> >
> > I don't see how you could use this approach to make a really robust, failure
> > proof storage system, but it might appear to work most of the time for most
> > people :-)
> >     
>
> You just do a write barrier after doing all the filesystem writing, and 
> you return with the guarantee that all the writes the filesystem did are 
> actually on disk.
>
>   
In this case, you have not gained anything  - same number of barrier 
operations/cache flushes and looser semantics for the transactions?
> No gray areas. No questions. No "might appear to work". 
>
> Sure, there might be other writes that got flushed _too_, but nobody 
> cares. If you have a crash later on, that's always true - you don't get 
> crashes at nice well-defined points.
>
> 			Linus
>   
This is pretty much how write barriers work today - you carry down other 
transactions (even for other partitions on the same disk) with you...

ric

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