[Bug 1532125] [NEW] lxc-clone: Use btrfs backing store if original container does

[Martin Pitt]

Public bug reported:

lxc-clone documents the backingstore option as "Select a different
backing store for the new container. By default the same as the original
container's is used.". This sounds like if the original container uses
btrfs, the cloned container should as well.

I have the LXC data dir on btrfs, and did lxc-create -n adt-xenial -B
best. This correctly selected btrfs backing store:

ID 293 gen 16369 top level 5 path lxc/adt-xenial/rootfs

However, cloning is slow:

$ time sudo lxc-clone -o adt-xenial -n x1
Created container x1 as copy of adt-xenial

real	0m9.417s
user	0m1.359s
sys	0m4.436s

It did create a new btrfs snapshot:
ID 297 gen 16372 top level 5 path lxc/x1/rootfs

but apparently not as a copy of the original  lxc/adt-xenial/rootfs one.

If I explicitly select btrfs as backingstore, it works as intended:

$ time sudo lxc-clone -B btrfs -o adt-xenial -n x1
Created container x1 as copy of adt-xenial

real	0m0.136s
user	0m0.004s
sys	0m0.012s

However, for scripts this is impractical, as it's not quite obvious how
to determine whether the original container uses btrfs backing store,
and "-B best" (which works for lxc-create) does not work for lxc-clone.
So it seems lxc-clone does not actually use "the same -B as the original
container"?

I haven't tested this for other backing stores.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: lxc 1.1.5-0ubuntu3
ProcVersionSignature: Ubuntu 4.3.0-5.16-generic 4.3.3
Uname: Linux 4.3.0-5-generic x86_64
ApportVersion: 2.19.3-0ubuntu2
Architecture: amd64
CurrentDesktop: Unity
Date: Fri Jan  8 10:10:03 2016
EcryptfsInUse: Yes
SourcePackage: lxc
UpgradeStatus: No upgrade log present (probably fresh install)
defaults.conf:
 lxc.network.type = veth
 lxc.network.link = lxcbr0
 lxc.network.flags = up
 lxc.network.hwaddr = 00:16:3e:xx:xx:xx
lxc.conf: lxc.lxcpath = /srv/lxc

** Affects: lxc (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: amd64 apparmor apport-bug xenial

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Title:
  lxc-clone: Use btrfs backing store if original container does

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