Hugo Mills posted on Tue, 22 Dec 2015 20:30:41 +0000 as excerpted: > On Mon, Dec 21, 2015 at 09:28:37AM +0000, Filipe Manana wrote: >> On Mon, Dec 21, 2015 at 8:14 AM, Hugo Mills <hugo@xxxxxxxxxxxxx> wrote: >> > On Sun, Dec 20, 2015 at 07:26:07PM -0600, Donald Pearson wrote: >> >> I read an implication in a different thread that defrag and >> >> autodefrag behave differently in that autodefrag is more snapshot >> >> friendly for COW data. >> > Correct. >> How is that so? Snapshot-aware defrag was disabled almost 2 years ago, >> and that piece of code is used both by a "manual" defrag (ioctl) and by >> automatic defrag. > Then I realised that actually, we're both right. :) > > If autodefrag behaves as you say (and I'm sure it does, since you > know the code), then when you write data to a file, a piece of the file > around the write(s) can be copied and written elsewhere by autodefrag. > That's going to be the same as the manual defrag. However, > it's a relatively small separation effect that's likely lost in the > noise. If you rewrite the whole file, then it'll separate the file > completely -- which is what manual defrag does -- but then, you'd expect > that behaviour without autodefrag, too. > > So, ultimately, autodefrag *does* separate reflink copies, but only > near where writes are made to the file, so the overall effect is very > similar to the effect you'd have from writing to the file without > autodefrag. > > And I am now better informed than I was before. :) And now so are we. Thanks, Hugo. I /knew/ there had to be a good explanation, as what you were saying just didn't fit in with what I knew. Now I have that explanation, and it makes perfect sense. =:^) -- 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 majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
