On Tue, Mar 2, 2010 at 3:02 AM, Luca Berra <bluca@xxxxxxxxxx> wrote: > On Tue, Mar 02, 2010 at 02:04:47AM -0800, Michael Evans wrote: >> >> On Mon, Mar 1, 2010 at 11:36 PM, Luca Berra <bluca@xxxxxxxxxx> wrote: >>> >>> On Tue, Mar 02, 2010 at 04:01:00PM +1100, Neil Brown wrote: >>>>> >>>>> Disk 1 contains A and B in the correct order, it is missing C and B' >>>>> but >>>>> we >>>>> dont care, app should be able to recover from a crash >>>>> >>>>> Disk 2 contains A and B', but they are wrongly ordered because C is >>>>> missing >>>>> >>>>> If in the above case A and C are data blocks and B contains a journal >>>>> related to A and C, booting from disk 2 could result in inconsistent >>>>> data. >>>>> >>>>> can the above really happen? >>>>> would using barriers remove the above concern? >>>>> am i missing something else? >>>> >>>> These is no inconsistency here that a filesystem would not equally >>>> expect >>>> from a single device. >>>> After the crash-while-writing B', it should expect to see either B or >>>> B', >>>> and it does, depending on which device is primary. >>>> >>>> Nothing to see here. >>> >>> I will try to explain better, >>> the problem is not related to the confusion between B or B' >>> >>> the problem is that on one disk we have B' _without_ C. >>> >> You're demanding full atomic commits; this is precisely what journals >> and /barriers/ are for. >> >> Are you are bypassing them in a quest for performance and paying for >> it on crashes? >> Or is this a hardware bug? >> Or is it some glitch in the block device layering leading to barrier >> requests not being honored? > > I just asked for confirmation that with /barriers/ the scenario above > would not happen. > > L. > > -- > Luca Berra -- bluca@xxxxxxxxxx > Communication Media & Services S.r.l. > /"\ > \ / ASCII RIBBON CAMPAIGN > X AGAINST HTML MAIL > / \ > -- > To unsubscribe from this list: send the line "unsubscribe linux-raid" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html > Yes, obviously atomic commits require barriers. Older hardware and operating systems that didn't allow any form of buffering or out of order operations (hardware can re-arrange commits internally now) inherently had a barrier between every operation. Modern devices and systems have so many layers of interacting buffers with operation re-ordering to optimize throughput that such predictability is lacking unless explicitly requested via the form of a barrier. -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
