Hans van Kranenburg posted on Sun, 05 Feb 2017 22:55:42 +0100 as excerpted: > On 02/05/2017 10:42 PM, Alexander Tomokhov wrote: >> Is it possible, having two drives to do raid1 for metadata but keep >> data on a single drive only? > > Nope. > > Would be a really nice feature though... Putting metadata on SSD and > bulk data on HDD... FWIW, it's on the list to be implemented in the future, but there's a lot more things on that list than devs working on btrfs, and the feature development and stabilization trend is that features often take much longer than anticipated, particularly to properly stabilize (to the level of the rest of btrfs, which is in general stabilizing but not yet fully stable, so stabilization is relative, here), which has slowed down the pipeline of new features due to the devs having their hands full stabilizing currently done but not yet properly stabilized features. So if they're not working on it yet, chances are it's going to be at /least/ three years to usably stable, and realistically, they may not even start working on it until perhaps 5-7 years out, so it could easily be another decade out for such a feature to properly stabilize. And given that the practical forecast horizon for free software is around five years, because enough unexpected happens in that time to make trying to predict in any worthwhile detail further out pretty much a fool's errand, effectively, it's at or outside the prediction horizon, so in practice, there's no telling /when/ it might be implemented and then stabilized enough to use, only that it /is/ on the list for "someday". Meanwhile, FWIW, my feature of choice, N-way-mirroring, has long been scheduled for "right after raid56 mode". Well, raid56 mode first wasn't introduced until long after originally scheduled (3.6 or earlier), then when it was it was runtime-only, the maintenance and recovery tools weren't there yet, then something like two years later they were ready so it was in theory feature-complete (in 3.19) but not yet stabilized, then in the stabilization process, some serious implementation bugs were found such that it can't yet be recommended and may yet require a full rewrite... so I'm still waiting for N-way-mirroring, which was supposed to follow shortly after raid56 mode. And given that raid56 mode might now take another couple years to stabilize (and it could be longer than that), I'm not expecting N-way- mirroring for another three years or so, so even it could well be five years out to usable stability. And it has long been at least in the known queue for after raid56 mode, and hasn't really been even started yet, so features on the list, but not yet actually on the queue, again, who knows, but five years out at least is a reasonable bet, and that's prediction horizon, so indeed, there's no way to really even predict, at this point. But I do know all about waiting, by now. I've learned, and am continuing to learn, all about patience! =:^] -- 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
