Rich Freeman posted on Thu, 15 Oct 2015 12:33:56 -0400 as excerpted: > On Wed, Oct 14, 2015 at 10:47 PM, Zygo Blaxell > <ce3g8...@umail.furryterror.org> wrote: >> >> I wouldn't describe dedup+defrag as unsafe. More like insane. You >> won't lose any data, but running both will waste a lot of time and >> power. Either one is OK without the other, or applied to >> non-overlapping sets of files, but they are operations with opposite >> results. > > That is probably why I disabled it then. I now recall past discussion > that defragging a file wasn't snapshot-aware, though I thought that was > fixed.
Well, snapshot-aware defrag was indeed released... in 3.9 IIRC (it's on the wiki)... Unfortunately, it didn't scale well /at/ /all/, as in spending half a day defragging just a handful of blocks. (Worst-case was many thousands of snapshots, with quotas enabled -- and this was back before the scalability work and quota code rewrites, so it was really **REALLY** bad!!111, and could be more like a day on a single block!) Obviously that simply didn't work /at/ /all/, so in ordered to have a defrag that was at least semi-usable, snapshot awareness was disabled again, making things /so/ much simpler, but of course dramatically increasing the possible downsides of actually running defrag. The disabling was in 3.12, IIRC, so defrag was only snapshot-aware for three kernel cycles, about seven months' worth of current development. Since then defrag's snapshot-awareness has remained disabled, tho AFAIK the plan remains to enable it again, if/when the devs are satisfied that they've made the code efficient and scalable enough that it's still going to be actually runnable in practical time, into (I'd say) 5-digits worth of snapshots, anyway. But that's going to necessarily involve stable quota code, and the quota code has been a continued problem tho there's some hope that it's coming together now, so it's anyone's guess when even the quota-code prerequisite is complete, let along if/when the scalability will then be reasonable enough, or if more work will still need to be done there... or if it'll _ever_ be practical to turn it back on. -- Duncan - List replies preferred. No HTML msgs. "Every nonfree program has a lord, a master -- and if you use the program, he is your master." Richard Stallman -- 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
