linux-kernel - Re: Poor interactive performance with I/O loads with fsync()ing

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Date:	Mon, 12 Apr 2010 01:43:44 +0200
From:	"Hans-Peter Jansen" <hpj@...la.net>
To:	linux-kernel@...r.kernel.org
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Andi Kleen <andi@...stfloor.org>, Avi Kivity <avi@...hat.com>,
	Ben Gamari <bgamari.foss@...il.com>,
	Arjan van de Ven <arjan@...radead.org>, tytso@....edu,
	npiggin@...e.de, Ingo Molnar <mingo@...e.hu>,
	Ruald Andreae <ruald.a@...il.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Olly Betts <olly@...vex.com>,
	martin f krafft <madduck@...duck.net>
Subject: Re: Poor interactive performance with I/O loads with fsync()ing

On Sunday 11 April 2010, 23:54:34 Thomas Gleixner wrote:
> On Sun, 11 Apr 2010, Andi Kleen wrote:
> > > XFS does not do much better. Just moved my VM images back to ext for
> > > that reason.
> >
> > Did you move from XFS to ext3?  ext3 defaults to barriers off, XFS on,
> > which can make a big difference depending on the disk. You can
> > disable them on XFS too of course, with the known drawbacks.
> >
> > XFS also typically needs some tuning to get reasonable log sizes.
> >
> > My point was merely (before people chime in with counter examples)
> > that XFS/btrfs/jfs don't suffer from the "need to sync all transactions
> > for every fsync" issue. There can (and will be) still other issues.
>
> Yes, I moved them back from XFS to ext3 simply because moving them
> from ext3 to XFS turned out to be a completely unusable disaster.
>
> I know that I can tweak knobs on XFS (or any other file system), but I
> would not have expected that it sucks that much for KVM with the
> default settings which are perfectly fine for the other use cases
> which made us move to XFS.

Thomas, what Andi was merely turning out, is that xfs has a really 
concerning different default: barriers, that hurts with fsync(). 

In order to make a fair comparison of the two, you may want to mount xfs 
with nobarrier or ext3 with barrier option set, and _then_ check which one 
is sucking less.

I guess, that outcome will be interesting for quite a bunch of people in the 
audience (including me¹).

Pete

¹) while in transition of getting rid of even suckier technology junk like
   VMware-Server - but digging out a current², but _stable_ kernel release
   seems harder then ever nowadays. 
²) with operational VT-d support for kvm
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