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 redhat.com :) > > 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? -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
