Christoph Anton Mitterer posted on Mon, 14 Dec 2015 03:46:01 +0100 as excerpted:
>> Same here. In fact, my most anticipated feature is N-way-mirroring, > Hmm ... not totally sure about that... > AFAIU, N-way-mirroring is what currently the currently wrongly called > RAID1 is in btrfs, i.e. having N replicas of everything on M devices, > right? > In other words, not being a N-parity-RAID and not guaranteeing that > *any* N disks could fail, right? No. N-way-mirroring, at least in simplest form (as in md/raid1) is N replicas on N devices, so loss of N-1 devices is permitted without loss of data. Normally the best thing about this is that unlike parity, once the general support is in, you can increase redundancy at will, with guaranteed device-loss protection of as many devices as you care to insure against. At one point with somewhat old devices that I didn't particularly trust any more and because I had them from a previous raid6 setup, I was running 4-way-md/raid1. Of course with md/raid1, the problem is lack of any sort of data integrity assurance, even scrubbing just arbitrarily chooses one and in the case of difference, simply copies that to the others, not even plurality-vote most authoritative version. With btrfs checksumming, the value of N-way-mirroring is increased dramatically, since it allows individual block verification and fallback, as opposed to whole-device-loss. While my own sweet-spot balance will tend to be three-way, avoiding the "if one copy is bad (perhaps because of a device that's known failing/ failed), you better /hope/ your only remaining copy is good" problem of the present two-way-only solution, I could easily see people finding value in 4/5/6-way mirroring as well. And of course if that is extended to raid10, three-way-mirroring, two-way- striping, on six total devices, would be my preferred, over the three-way- striped, two-way-mirrored, that's the only current choice for six-device btrfs raid10. -- 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
