Bug 1208566

Description of problem:

Minor issues, since a series of qemu-img commands can achieve the same, but it might be nice to consider single-command options rather than requiring users to know how to chain a sequence of commands.

QMP is able to shorten a backing file chain of A <- B <- C down to A <- C by committing B into A and leaving content unchanged in C (requires writing to both A [committed data] and C [metadata]).  qemu-img, however, requires two commands (commit B, rebase -u -b A C)

On a related vein, 'qemu-img commit C' is useful for emptying out the contents of C but preserving the chain (changing the point in time that C is tracking from).  However, this only works into a single backing file at a time.  It would be nice to be able to do a multi-file commit as efficiently as possible (committing C into B, then B into A, then rebasing C onto A, requires wasted sector copies).  That is, the ability to go from:
A 1111   # snapshot sees 1111
B -22-   # snapshot sees 1221
C --33   # guest sees 1233

to:
A 1233   # snapshot sees 1233
C ----   # guest sees 1233

in a single qemu-img invocation might be useful.


Version-Release number of selected component (if applicable):
still present upstream as of 2.3

How reproducible:
100%

Steps to Reproduce:
1. given a top level file, try and use qemu-img to shorten the chain by committing an intermediate image into the base
2.
3.

Actual results:
no single command exists, user must issue multiple qemu-img commands

Expected results:
a single qemu-img command can do it all

Additional info:
see also this thread: https://lists.gnu.org/archive/html/qemu-devel/2015-04/msg00276.html