On Sun, 2016-06-05 at 09:36 -0600, Chris Murphy wrote: > That's ridiculous. It isn't incorrect to refer to only 2 copies as > raid1. No, if there are only two devices then not. But obviously we're talking about how btrfs does RAID1, in which even with n>2 devices there are only 2 copies - that's incorrect.
> You have to explicitly ask both mdadm Aha, and which option would that be? > and lvcreate for the > number of copies you want, it doesn't automatically happen. I've said that before, but at least it allows you to use the full number of disks, so we're again back to that it's closer to the original and common meaning of RAID1 than what btrfs does. > The man > page for mkfs.btrfs is very clear you only get two copies. I haven't denied that... but one shouldn't use terms that are commonly understood in a different mannor and require people to read all the small printed. One could also have changed it's RAID0 with RAID1, and I guess people wouldn't be too delighted if the excuse was "well it's in the manpage". > > > Well I'd say, for btrfs: do away with the term "RAID" at all, use > > e.g.: > > > > linear = just a bunch of devices put together, no striping > > basically what MD's linear is > Except this isn't really how Btrfs single works. The difference > between mdadm linear and Btrfs single is more different in behavior > than the difference between mdadm raid1 and btrfs raid1. So you're > proposing tolerating a bigger difference, while criticizing a smaller > one. *shrug* What's the big difference? Would you care to explain? But I'm happy with "single" either, it just doesn't really tell that there is no striping, I mean "single" points more towards "we have no resilience but only 1 copy", whether this is striped or not. > If a metaphor is going to be used for a technical thing, it would be > mirrors or mirroring. Mirror would mean exactly two (the original and > the mirror). See lvcreate --mirrors. Also, the lvm mirror segment > type > is legacy, having been replaced with raid1 (man lvcreate uses the > term > raid1, not RAID1 or RAID-1). So I'm not a big fan of this term. Admittedly, I didn't like the "mirror(s)" either... I was just trying to show that different names could be used that are already a bit better. > > striped = basically what RAID0 is > > lvcreate uses only striped, not raid0. mdadm uses only RAID0, not > striped. Since striping is also employed with RAIDs 4, 5, 6, 7, it > seems ambiguous even though without further qualification whether > parity exists, it's considered to mean non-parity striping. The > ambiguity is probably less of a problem than the contradiction that > is > RAID0. Mhh,.. well or one makes schema names that contain all possible properties of a "RAID", something like: replicasN-parityN-[not]striped SINGLE would be something like "replicas1-parity0-notstriped". RAID5 would be something like "replicas0-parity1-striped". > > And just mention in the manpage, which of these names comes closest > > to > > what people understand by RAID level i. > > It already does this. What version of btrfs-progs are you basing your > criticism on that there's some inconsistency, deficiency, or > ambiguity > when it comes to these raid levels? Well first, the terminology thing is the least serious issue from my original list ;-) ... TBH I don't know why such a large discussion came out of that point. Even though I'm not reading along all mails here, we have probably at least every month someone who wasn't aware that RAID1 is not what he assumes it to be. And I don't think these people can be blamed for not RTFM, because IMHO this is a term commonly understood as mirror all available devices. That's how the original paper describes it, it's how Wikipedia describes it and all other sources I've ever read to the topic. > The one that's unequivocally > problematic alone without reading the man page is raid10. The > historic > understanding is that it's a stripe of mirrors, and this suggests you > can lose a mirror of each stripe i.e. multiple disks and not lose > data, which is not true for Btrfs raid10. But the man page makes that > clear, you have 2 copies for redundancy, that's it. Yes, same basic problem. > On the CLI? Not worth it. If the user is that ignorant, too bad, use > a > GUI program to help build the storage stack from scratch. I'm really > not sympathetic if a user creates a raid1 from two partitions of the > same block device anymore than if it's ultimately the same physical > device managed by a device mapper variant. Well one I have no strong opinion on that... if testing for it (or at least simple cases) would be easy, why not. Not every situation may be as easily visible as creating a RAID1 on /dev/sda1 and /dev/sda2. One may use LABELs, or UUIDs and accidentally catch the wrong, and in such cases a check may help. Cheers, Chris.
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