Re: [PATCH 0/4] (RESEND) ext3[34] barrier changes

From: Andrew Morton
Date: Fri May 16 2008 - 16:59:32 EST

On Fri, 16 May 2008 15:53:31 -0500
Eric Sandeen <sandeen@xxxxxxxxxx> wrote:

> Andrew Morton wrote:
> > On Fri, 16 May 2008 14:02:46 -0500
> > Eric Sandeen <sandeen@xxxxxxxxxx> wrote:
> >
> >> A collection of patches to make ext3 & 4 use barriers by
> >> default, and to call blkdev_issue_flush on fsync if they
> >> are enabled.
> >
> > Last time this came up lots of workloads slowed down by 30% so I
> > dropped the patches in horror.
> I actually did a bit of research and found the old thread, honestly. I
> thought this might not be a shoo-in. :) Seems worth hashing out, though.
> > I just don't think we can quietly go and slow everyone's machines down
> > by this much. The overhead of journalling is already pretty horrid.
> But if journali[zi]ng guarantees are thrown out the window by volatile
> caches on disk, why bother with the half-solution? Slower while you
> run, worthless when you lose power? Sounds like the worst of both
> worlds. (well, ok, experience shows that it's not worthless in practice...)
> > If we were seeing a significant number of "hey, my disk got wrecked"
> > reports which attributable to this then yes, perhaps we should change
> > the default. But I've never seen _any_, although I've seen claims that
> > others have seen reports.
> Hm, how would we know, really? What does it look like? It'd totally
> depend on what got lost... When do you find out? Again depends what
> you're doing, I think. I'll admit that I don't have any good evidence
> of my own. I'll go off and do some plug-pull-testing and a benchmark or
> two.
> But, drive caches are only getting bigger, I assume this can't help. I
> have a hard time seeing how speed at the cost of correctness is the
> right call...

Yeah, it's all so handwavy. The only thing which isn't handwavy is
that performance hit.

> > There are no happy solutions here, and I'm inclined to let this dog
> > remain asleep and continue to leave it up to distributors to decide
> > what their default should be.
> >
> > Do we know which distros are enabling barriers by default?
> SuSE does (via patch for ext3). Red Hat & Fedora don't, and install by
> default on lvm which won't pass barriers anyway. So maybe it's
> hypocritical to send this patch from :)
> And as another "who uses barriers" datapoint, reiserfs & xfs both have
> them on by default.
> I suppose alternately I could send another patch to remove "remember
> that ext3/4 by default offers higher data integrity guarantees than
> most." from Documentation/filesystems/ext4.txt ;)

We could add a big scary printk at mount time and provide a document?
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