On Sun, 04 May 2014 09:27:10 +0200 Brendan Hide <bren...@swiftspirit.co.za> wrote:
> On 2014/05/04 05:27 AM, Duncan wrote: > > Russell Coker posted on Sun, 04 May 2014 12:16:54 +1000 as > > excerpted: > > > >> Are there any plans for a feature like the ZFS copies= option? > >> > >> I'd like to be able to set copies= separately for data and > >> metadata. In most cases RAID-1 provides adequate data protection > >> but I'd like to have RAID-1 and copies=2 for metadata so that if > >> one disk dies and another has some bad sectors during recovery I'm > >> unlikely to lose metadata. > > Hugo's the guy with the better info on this one, but until he > > answers... > > > > The zfs license issues mean it's not an option for me and I'm thus > > not familiar with its options in any detail, but if I understand > > the question correctly, yes. > > > > And of course since btrfs treats data and metadata separately, it's > > extremely unlikely that any sort of copies= option wouldn't be > > separately configurable for each. > > > > There was a discussion of a very nice multi-way-configuration > > schema that I deliberately stayed out of as both a bit above my > > head and far enough in the future that I didn't want to get my > > hopes up too high about it yet. I already want N-way-mirroring so > > bad I can taste it, and this was that and way more... if/when it > > ever actually gets coded and committed to the mainline kernel > > btrfs. As I said, Hugo should have more on it, as he was active in > > that discussion as it seemed to line up perfectly with his area of > > interest. > > > The simple answer is yes, this is planned. As Duncan implied, > however, it is not on the immediate roadmap. Internally we appear to > be referring to this feature as "N-way redundancy" or "N-way > mirroring". > > My understanding is that the biggest hurdle before the primary devs > will look into N-way redundancy is to finish the Raid5/6 > implementation to include self-healing/scrubbing support - a critical > issue before it can be adopted further. Well, there's N-way-mirroring, which /is/ on the roadmap for fairly soon (after raid56 completion), and which is the feature I've been heavily anticipating ever since I first looked into btrfs and realized that raid1 didn't include it already, but what I was referring to above was something much nicer than that. As I said I don't understand the full details, Hugo's the one that can properly answer there, but the general idea (I think) is the ability to three-way specify N-copies, M-parity, S-stripe, possibly with near/far-layout specification like md/raid's raid10, as well. But Hugo refers to it with three different letters, cps copies/parity/stripes, perhaps? That doesn't look quite correct... But that at least has the potential to be /so/ nice, and possibly also /so/ complicated, that I'm deliberately avoiding looking too much at the details as it's far enough out and may in fact never get fully implemented that I don't want to spoil my enjoyment of (relatively, compared to that) simple N-way-mirroring when it comes. And more particularly, I really /really/ hope they don't put off a reasonably simple and (hopefully) fast implementation of N-way-mirroring as soon as possible after raid56 completion, because I really /really/ want N-way-mirroring, and this other thing would certainly be extremely nice, but I'm quite fearful that it could also be the perfect being the enemy of the good-enough, and btrfs already has a long history of features repeatedly taking far longer to implement than originally predicted, which with something that potentially complex, I'm very afraid could mean a 2-5 year wait before it's actually usable. And given how long I've been waiting for the simple-compared-to-that N-way-mirroring thing and how much I anticipate it, I just don't know what I'd do if I were to find out that they were going to work on this "perfect" thing instead, with N-way-mirroring being one possible option with it, but that as a result, given the btrfs history to date, it'd very likely be a good five years before I could get the comparatively simple N-way-mirroring (or even, for me, just a specific 3-way-mirroring to compliment the specific 2-way-mirroring that's already there) that's all I'm really asking for. So I guess you can see why I don't want to get into the details of the more fancy solution too much, both as a means of protecting my own sanity, and to hopefully avoid throwing the 3-way-mirroring that's my own personal focal point off the track. So Hugo's the one with the details, to the extent they've been discussed at least, there. -- Duncan - No HTML messages please, as they are filtered as spam. "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
