On Apr 24, 2014, at 9:55 AM, Hugo Mills <hugo@xxxxxxxxxxxxx> wrote: > On Thu, Apr 24, 2014 at 09:23:28AM -0600, Chris Murphy wrote: >> >> >> I don't understand the btrfs send -c <clone-src> man page text, or really even the use case. In part this is what it says: >> >>> You must not specify clone sources unless you >>> guarantee that these snapshots are exactly in the same state on both >>> sides, the sender and the receiver. >> >> If the snapshots are the same on both sides, then why would I be using clone in the first place? > > To copy over another snapshot which shares data with them. > >>> -c <clone-src> Use this snapshot as a clone source for an >>> incremental send (multiple allowed) >> >> Incremental send implies the sender and receiver are not in the same state now, but will be after the command is executed. Is one, or both, snapshots rw for -c? >> >> Anyway, I'm lost on the specifics, but clearly I'm even lost when it comes to the basic difference between -p and -c. > > (Note: I've not actually tried the second case in what follows, but > it's what I think is going on. This may be subject to corrections.) > > OK, call the sending system "S" and the receiving system "R". Let's > say we've got three subvolumes on S: > > S:A2, the current /home (say) > S:A1, a snapshot of an earlier version of S:A2 > S:B, a separate subvolume that's had some CoW copies of files in both > S:A1 and S:A2 made into it. > > If we send S:A1 to R, then we'll have to send the whole thing, > because R doesn't have any subvolumes yet. > > If we now want to send S:A2 to R, then we can use -p S:A1, and it > will send just the differences between those two. This means that the > send stream can potentially ignore a load of the metadata as well as > the data. It's effectively saying, "you can clone R:A1, then do these > things to it to get R:A2". > > If we now want to send S:B to R, then we can use -c S:A1 -c S:A2. OK this makes sense now, thanks. Does the use of -c always require at least two -c instances? Is there an example where -c is used once? From the man page I'm not groking that there must be at least two -c's. > I'm trying to think of a case where -c is useful that doesn't > involve someone having done cp --reflink=always between subvolumes, > but I can't. OK. > So, I think the summary is: > > * Use -p to deal with parent-child reflinks through snapshots > * Use -c to specify other subvolumes (present on both sides) that > might contain reflinked data I think the key is that -c implies a minimum of five subvolumes: two subvolumes on the source, which have (identical) counterparts on the destination (that's four subvolumes), and then one additional somehow related subvolume B on the source that I want on the destination. Whereas -p implies three subvolumes (one on the source which is the parent, its counterpart on the destination, and a child on the source which I want on the destination). I necessarily must understand the relationship among them in order to get the desired incremental result on the destination. Chris Murphy -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
