The implementation of qemu-img convert is (a) messy, (b) buggy, and
(c) less efficient than possible. The changes required to beat some
sense into it are massive enough that incremental changes would only
make my and the reviewers' life harder. So throw it away and reimplement
it from scratch.
Let me give some examples what I mean by messy, buggy and inefficient:
(a) The copying logic of qemu-img convert has two separate branches for
compressed and normal target images, which roughly do the same -
except for a little code that handles actual differences between
compressed and uncompressed images, and much more code that
implements just a different set of optimisations and bugs. This is
unnecessary code duplication, and makes the code for compressed
output (unsurprisingly) suffer from bitrot.
The code for uncompressed ouput is run twice to count the the total
length for the progress bar. In the first run it just takes a
shortcut and runs only half the loop, and when it's done, it toggles
a boolean, jumps out of the loop with a backwards goto and starts
over. Works, but pretty is something different.
(b) Converting while keeping a backing file (-B option) is broken in
several ways. This includes not writing to the image file if the
input has zero clusters or data filled with zeros (ignoring that the
backing file will be visible instead).
It also doesn't correctly limit every iteration of the copy loop to
sectors of the same status so that too many sectors may be copied to
in the target image. For -B this gives an unexpected result, for
other images it just does more work than necessary.
Conversion with a compressed target completely ignores any target
backing file.
(c) qemu-img convert skips reading and writing an area if it knows from
metadata that copying isn't needed (except for the bug mentioned
above that ignores a status change in some cases). It does, however,
read from the source even if it knows that it will read zeros, and
then search for non-zero bytes in the read buffer, if it's possible
that a write might be needed.
This reimplementation of the copying core reorganises the code to remove
the duplication and have a much more obvious code flow, by essentially
splitting the copy iteration loop into three parts:
1. Find the number of contiguous sectors of the same status at the
current offset (This can also be called in a separate loop before the
copying loop in order to determine the total sectors for the progress
bar.)
2. Read sectors. If the status implies that there is no data there to
read (zero or unallocated cluster), don't do anything.
3. Write sectors depending on the status. If it's data, write it. If
we want the backing file to be visible (with -B), don't write it. If
it's zeroed, skip it if you can, otherwise use bdrv_write_zeroes() to
optimise the write at least where possible.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
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