Re: Proposal: B_ARRIER (addresses wapbl performance?)

[Manuel Bouyer <bouyer%antioche.eu.org@localhost>]

On Tue, Dec 09, 2008 at 03:04:15PM +1100, Daniel Carosone wrote:
> [...]
> A general comment from reading this thread a while ago, now that it
> has been awoken again:  people seem to be arguing at cross
> purposes.  
> 
> There are two features being discussed and positions being taken, but
> perhaps participants are missing that they're complementary:
> 
>    One side argues that using FUA for log writes is essential, because
>    for those you want to know the log is written, without undue
>    overhead from flushing other pending writes.
> 
>    The other side argues that using ordered tags is essential to know
>    that some set of writes has completed before some other set of
>    writes are processed, and likewise that the completion of the
>    latter also reliably implies the completion of the former.
> 
> Nobody seems to have pointed out that they work together: 
> 
>    log writes should be done with FUA, for minimal-latency commitment
>    of transactions, 
> 
>    while ordered completion of other data+metadata writes is vital for
>    updating the filesystem proper, and so knowing when the log tail
>    can be freed again.  

This makes a lot of sense. I had trouble figuring out how a FUA for journal,
without a FUA for metadata could work ...
I'm not even sure FUA is necessery, if the ordered tag is working
properly (i.e. the device doesn't merge different writes in its cache,
which would break ordering).

-- 
Manuel Bouyer <bouyer%antioche.eu.org@localhost>
     NetBSD: 26 ans d'experience feront toujours la difference
--