Hi,
> I admit I haven't completely understood what you're trying to achieve. You can
> only receive an incremental stream if the internal (!) file system state on the
> receiver's side is the same as on the sender's.
Understood, and the internal states are identical, except for the
snapshot metadata.
This executable example that shows the problem:
# Create two throwaway filesystems /tmp/img1 and /tmp/img2
# and mount them at /tmp/mp1 and /tmp/mp2
for i in 1 2; do
truncate -s 1G /tmp/img$i
loop=`losetup -f --show /tmp/img$i`
mkfs.btrfs "$loop"
losetup -d "$loop"
mkdir /tmp/mp$i
mount /tmp/img$i /tmp/mp$i
done
# Create a subvol on the first fs, populate it
btrfs sub create /tmp/mp1/foo
echo 1 >/tmp/mp1/foo/1
# Make an ro snapshot of the subvol, transfer it to the 2nd fs
btrfs sub snap -r /tmp/mp1/foo /tmp/mp1/foo.1
btrfs send /tmp/mp1/foo.1 | btrfs receive -v /tmp/mp2
# Make an rw snapshot of the transferred snapshot on the 2nd fs,
# add something to this rw snapshot
btrfs sub snap /tmp/mp2/foo.1 /tmp/mp2/foo
echo 2 >/tmp/mp2/foo/2
# Make an ro snapshot of the added-to snapshot
btrfs sub snap -r /tmp/mp2/foo /tmp/mp2/foo.2
# Now try to transfer it back in the other direction
btrfs send -p /tmp/mp2/foo.1 /tmp/mp2/foo.2 |
btrfs receive -v /tmp/mp1
# "ERROR: could not find parent subvolume"
# even though both fs have the same foo.1
As I understand it, if "clone" is a clone/parent source, btrfs send
transmits
clone.uuid
clone.ctransid
and btrfs receive searches for a subvol "sub" with
sub.received_uuid == clone.uuid and sub.stransid == clone.ctransid.
But because the direction of the transfer has changed, it can't find
anything. It would work if btrfs send was modified to *additionally*
transmit
clone.received_uuid
clone.stransid
and btrfs receive to do a *fallback* search for a subvol with
sub.uuid == clone.received_uuid and sub.ctransid == clone.stransid
However, since I'm really not familiar with btrfs code, it was easier
for me to just make a rw snapshot of mp1/foo.1 and modify its metadata:
# can't modify the info about a ro snapshot
btrfs sub snap /tmp/mp1/foo.1 /tmp/mp1/foo.1rw
# mp1/foo.1rw.received_uuid := mp2/foo.1.uuid
# mp1/foo.1rw.stransid := mp2/foo.1.ctransid
uu /tmp/mp2 foo.1 /tmp/mp1/foo.1rw
# !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
# !! Now we've made it look as if mp1/foo.1 was received from !!
# !! mp2/foo.1 when actually it was the other way around. !!
# !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
# Let's retry the original problematic command:
btrfs send -p /tmp/mp2/foo.1 /tmp/mp2/foo.2 |
btrfs receive -v /tmp/mp1
# It works fine! Hurray!
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