linux-kernel - Re: Linux 2.6.29

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Date:	Wed, 25 Mar 2009 20:41:17 +0000 (GMT)
From:	Hugh Dickins <hugh@...itas.com>
To:	Jens Axboe <jens.axboe@...cle.com>
cc:	Ric Wheeler <rwheeler@...hat.com>, Jeff Garzik <jeff@...zik.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Theodore Tso <tytso@....edu>, Ingo Molnar <mingo@...e.hu>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Arjan van de Ven <arjan@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Nick Piggin <npiggin@...e.de>, David Rees <drees76@...il.com>,
	Jesper Krogh <jesper@...gh.cc>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Linux 2.6.29

On Wed, 25 Mar 2009, Jens Axboe wrote:
> On Wed, Mar 25 2009, Ric Wheeler wrote:
> > Jens Axboe wrote:
> >>
> >> Another problem is that FLUSH_CACHE sucks. Really. And not just on
> >> ext3/ordered, generally. Write a 50 byte file, fsync, flush cache and
> >> wit for the world to finish. Pretty hard to teach people to use a nicer
> >> fdatasync(), when the majority of the cost now becomes flushing the
> >> cache of that 1TB drive you happen to have 8 partitions on. Good luck
> >> with that.
> >>   
> > And, as I am sure that you do know, to add insult to injury, FLUSH_CACHE  
> > is per device (not file system).
> >
> > When you issue an fsync() on a disk with multiple partitions, you will  
> > flush the data for all of its partitions from the write cache....
> 
> Exactly, that's what my (vague) 8 partition reference was for :-)
> A range flush would be so much more palatable.

Tangential question, but am I right in thinking that BIO_RW_BARRIER
similarly bars across all partitions, whereas its WRITE_BARRIER and
DISCARD_BARRIER users would actually prefer it to apply to just one?

Hugh
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