Am Wed, 9 Dec 2015 13:36:01 +0000 (UTC) schrieb Duncan <1i5t5.duncan@xxxxxxx>: > >> > 4) Duncan mentioned that defrag (and I guess that's also for > >> > auto- defrag) isn't ref-link aware... > >> > Isn't that somehow a complete showstopper? > > >> It is, but the one attempt at dealing with it caused massive data > >> corruption, and it was turned off again. > > IIRC, it wasn't data corruption so much, as massive scaling issues, > to the point where defrag was entirely useless, as it could take a > week or more for just one file. > > So the decision was made that a non-reflink-aware defrag that > actually worked in something like reasonable time even if it did > break reflinks and thus increase space usage, was of more use than a > defrag that basically didn't work at all, because it effectively took > an eternity. After all, you can always decide not to run it if you're > worried about the space effects it's going to have, but if it's going > to take a week or more for just one file, you effectively don't have > the choice to run it at all. > > > So... does this mean that it's still planned to be implemented some > > day or has it been given up forever? > > AFAIK it's still on the list. And the scaling issues are better, but > one big thing holding it up now is quota management. Quotas never > have worked correctly, but they were a big part (close to half, IIRC) > of the original snapshot-aware-defrag scaling issues, and thus must > be reliably working and in a generally stable state before a > snapshot-aware-defrag can be coded to work with them. And without > that, it's only half a solution that would have to be redone when > quotes stabilized anyway, so really, quota code /must/ be stabilized > to the point that it's not a moving target, before reimplementing > snapshot-aware-defrag makes any sense at all. > > But even at that point, while snapshot-aware-defrag is still on the > list, I'm not sure if it's ever going to be actually viable. It may > be that the scaling issues are just too big, and it simply can't be > made to work both correctly and in anything approaching practical > time. Time will tell, of course, but until then... I'd like to throw in an idea... Couldn't auto-defrag just be made "sort of reflink-aware" in a very simple fashion: Just let it ignore extents that are shared? That way you can still enjoy it benefits in a mixed-mode scenario where you are working with snapshots partly but other subvolumes are never taken snapshots of. Comments? -- -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
