On Fri, Jun 29, 2018 at 1:09 PM, Austin S. Hemmelgarn <ahferro...@gmail.com> wrote: > On 2018-06-29 11:15, james harvey wrote: >> >> On Thu, Jun 28, 2018 at 6:27 PM, Chris Murphy <li...@colorremedies.com> >> wrote: >>> >>> And an open question I have about scrub is weather it only ever is >>> checking csums, meaning nodatacow files are never scrubbed, or if the >>> copies are at least compared to each other? >> >> >> Scrub never looks at nodatacow files. It does not compare the copies >> to each other. >> >> Qu submitted a patch to make check compare the copies: >> https://patchwork.kernel.org/patch/10434509/ >> >> This hasn't been added to btrfs-progs git yet. >> >> IMO, I think the offline check should look at nodatacow copies like >> this, but I still think this also needs to be added to scrub. In the >> patch thread, I discuss my reasons why. In brief: online scanning; >> this goes along with user's expectation of scrub ensuring mirrored >> data integrity; and recommendations to setup scrub on periodic basis >> to me means it's the place to put it. > > That said, it can't sanely fix things if there is a mismatch. At least, not > unless BTRFS gets proper generational tracking to handle temporarily missing > devices. As of right now, sanely fixing things requires significant manual > intervention, as you have to bypass the device read selection algorithm to > be able to look at the state of the individual copies so that you can pick > one to use and forcibly rewrite the whole file by hand.
Absolutely. User would need to use manual intervention as you describe, or restore the single file(s) from backup. But, it's a good opportunity to tell the user they had partial data corruption, even if it can't be auto-fixed. Otherwise they get intermittent data corruption, depending on which copies are read. > A while back, Anand Jain posted some patches that would let you select a > particular device to direct all reads to via a mount option, but I don't > think they ever got merged. That would have made manual recovery in cases > like this exponentially easier (mount read-only with one device selected, > copy the file out somewhere, remount read-only with the other device, drop > caches, copy the file out again, compare and reconcile the two copies, then > remount the volume writable and write out the repaired file). -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
