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.
Note that S:B doesn't have any metadata in common with either of the
As, only data. This will send all of the metadata ("start with an
empty subvolume and do these things to it to get R:B"), but because
it's known to share data with some subvols on S, and those subvols
also exist on R, we can avoid sending that data again by simply
specifying where the data can be found and reflinked from on R.
So, if you have a load of snapshots, you can do one of two things
to duplicate all of them:
btrfs sub send <snap 0>
for n=1 to N
btrfs sub send -p <snap n-1> <snap n>
Or, in any order,
btrfs sub send <snap s1>
for n=1 to N
btrfs sub send -c <snap s1> -c <snap s2> -c <snap s3> ... <snap sn>
where each subvolume that's been sent before gets added as a -c to the
next send command. This second approach means that all possible
reflinks between subvolumes can be captured, but it will send all of
the metadata across each time. The first approach may lose some manual
reflink efficiency, but is better at sending only the necessary
changed metadata. You should be able to combine the two methods, I
think.
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. 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
Hugo.
--
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