Christoph Anton Mitterer posted on Mon, 28 Dec 2015 01:50:09 +0100 as excerpted:
> On Sun, 2015-12-27 at 07:09 +0000, Duncan wrote: >> raid1 mode > I wonder when that reaches my pain threshold... and I submit a patch > that renames it "notreallyraid1" in all places ;-) I've seen two responses to that, both correct, AFAIK. 1) Btrfs very specifically and deliberately uses *lowercase* raidN in part to make that distinction, as the btrfs variants are chunk-level (and designed so that at some point in the future they can be subvolume and/or file level), not device-level (and at that future point, not necessarily filesystem level either). As we've seen in discussion in other threads, for raid10 in particular, that makes a profound difference in robustness in the multi-device failure case. 2) Regarding btrfs raid1 and raid10's current very specific two-way- mirroring in particular, limiting to two-way-mirroring in the 3+ devices case is well within established definitions and historic usage. Apparently, the N-devices = N-way-mirroring usage is relatively new, arguably first popularized by Linux mdraid, after which various hardware raid suppliers also implemented it due to competitive pressure. But only two-way-mirroring is required by the RAID-1 definition. Even were that not the case, point #1, btrfs' very specific use of *lowercase* raid1, still covers the two-way-limitation case just as well as it covers the chunk-level case. That said, that the limited pair-mirroring btrfs implements even in the 3+ device case still meets formal RAID-1 definitions was originally news to me as well, however well I might now accept the fact. But once my earlier naive assumptions were corrected, the remaining clarification issues fell below my pain threshold. But for those for whom it's still very close to their pain threshold, due to the above a patch effectively doing s/raid1/notreallyraid1/g is unlikely to be accepted. Much more likely to be accepted would be a patch to the btrfs-balance and mkfs.btrfs manpages adding note, preferably accounting for the raid10 situation as well, explaining that btrfs raid (lowercase) isn't RAID (uppercase) in the the traditional sense, that it's chunk-scope not device-scope and that this has implications in for instance robustness in the raid10 multi-device failure case, and that both raid1 and raid10 are (currently) limited to two-way-mirroring. Meanwhile, for anyone considering writing that patch, I'd also strongly recommend that the two-way-mirroring wording is separated out, at least onto its own lines if not a separate paragraph, so it can be cleanly deleted and/or modified once N-way-mirroring is introduced as a feature, without having to rewrite the chunk-level and raid10 bit as well. -- 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
