On Thu, Nov 27, 2014 at 2:08 AM, Duncan <1i5t5.dun...@cox.net> wrote: > So, umm... kinda late now, but read that "copy" as if it had a footnote > attached, saying "Yes, I know it's not actual copy, it's two views of the > same thing using COW, but my point is, from the btrfs perspective it's a > copy, the "universally UNIQUE ID" no longer looks "unique" and thus no > longer can be properly called a UUID at all."
The copy is sort of a misnomer anyway because up until the computer age the copy was a derivative, a facsimile, like a photocopy. But a copy of a digital file is actually another original. Therein lies the problem with the LVM snapshot in this context, we don't want another original. We want a copy, as in we want something we know has been derived from something else, and therefore can be discriminated. And that's the same problem with subvolume UUIDs being "reused" when creating new Btrfs volumes, which have new volume UUIDs, from a Btrfs seed device. There are now multiple originals of those subvolumes, there's no distinguishing them by their UUID alone. > But... I'd still say LVM is "at fault" to the extent that anyone is, as > it /knows/ it's dealing with UUIDs because after all that's part of > what's /on/ what it's snapshotting, and it doesn't make any effort to > deal with the situation, despite the at least theoretical (and now in > fact) confusion that may occur when former UUIDs are no longer unique and > thus no longer UUIDs. Well RFC 4122 I don't think would say it's not a UUID, the uniqueness is only guaranteed at the time of UUID creation. And duplication isn't creation so it's not going to say these things are no longer UUIDs, they're just UUIDs that have been recycled. That RFC doesn't specify workflow, but if it did, I think it'd basically say "oh crap, why'd you go and do that?" After all a major point of UUIDs is that they are effectively unlimited in quantity, therefore a.) we don't need central registry to avoid (unintended) collisions because they're so uncommon, b.) we're encouraged to not be attached to specific UUIDs when in doubt just create another one. A very good example of WTF reusage of a UUID that irks me to no end is GNU parted devs decided to recycle the Microsoft Windows Basic Data partition type GUID for Linux partitions. It's like watching someone get run over by a zamboni with 50 feet of advance notice... -- Chris Murphy -- 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
