On Thu, Apr 16, 2020 at 5:51 PM Eric Blake <ebl...@redhat.com> wrote:
>
> Without this series, the process for copying one qcow2 image to
> another including all of its bitmaps involves running qemu and doing
> the copying by hand with a series of QMP commands. This makes the
> process a bit more convenient.
This seems good for copying an image chain from one storage to another,
but I think we need a similar --bitmaps option to qemu-img measure to make
this really useful.
Here is example use case showing how qemu-img measure is related:
Source chain:
/dev/vg1/base
/dev/vg1/top
Destination chain:
/dev/vg2/base
/dev/vg2/top
We create empty lvs with the same name on destination storage (/dev/vg2).
We measure the base lv using qemu-img measure for creating the target lv:
qemu-img measure -f qcow2 -O qcow2 /dev/vg1/base
lvcreate -L required_size /dev/vg2/base
qemu-img create -f qcow2 /dev/vg2/base 10g
For the top lv we use the current size of the source lv - I think we
should measure it instead but
I'm not sure if qemu-img measure supports measuring a single image in a chain
(maybe -o backing_file?).
lvcreate -L current_size /dev/vg2/top
qemu-img create -f qcow2 -b /dev/vg2/base -F qcow2 /dev/vg2/top 10g
And then convert the lvs one by one:
qemu-img convert -f qcow2 -O qcow2 -n --bitmaps /dev/vg1/base /dev/vg2/base
qemu-img convert -f qcow2 -O qcow2 -n --bitmaps -B /dev/vg2/base
/dev/vg1/top /dev/vg2/top
The first copy may fail with ENOSPC since qemu-img measure of the base
does not consider the
bitmaps in the required size.
So I think we need to add a similar --bitmaps option to qemu-img
measure, hopefully reusing the
same code to find and estimate the size of the bitmaps.
Maybe we can estimate the size using qemu-img info --bitmaps, but I
think the right way to
do this is in qemu-img measure.
We have also another use case when we collapsed an image chain to single image:
Source chain:
/dev/vg1/base
/dev/vg1/top
Destination:
/dev/vg2/collapsed
In this case we measure the size of the entire chain (/dev/vg1/base <-
/dev/vg1/top) and create
/dev/vg2/collapsed in the correct size, and then we convert the chain using:
qemu-img convert /dev/vg1/top /dev/vg2/collapsed
Currently we use this for exporting images, for example when creating
templates, or as a simple
backup. In this case we don't need to copy the bitmaps in the target
image - this is a new image
not used by any VM. Copying the bitmaps may also be non-trivial since
we may have the bitmaps
with the same names in several layers (e.g. result of live snapshot).
So I think using --bitmaps should be disabled when doing this kind of
convert. We can handle this
on our side easily, but I think this should fail or log a warning on
qemu-img, or require merging of
bitmaps with same names during the copy. I did not check if you
already handle this.
Finally we also have a use case when we copy the chain as is to new or
same storage, but
we create a new vm. In this case I don't think the backup history
makes sense for the new
vm, so we don't need to copy the bitmaps.
I will review the rest of the patches next week and can maybe give
this some testing.
Nir
> I still think that someday we will need a 'qemu-img bitmap' with
> various subcommands for manipulating bitmaps within an offline image,
> but in the meantime, this seems like a useful addition on its own.
>
> Series can also be downloaded at:
> https://repo.or.cz/qemu/ericb.git/shortlog/refs/tags/qemu-img-bitmaps-v1
>
> Eric Blake (3):
> blockdev: Split off basic bitmap operations for qemu-img
> qemu-img: Add convert --bitmaps option
> iotests: Add test 291 to for qemu-img convert --bitmaps
>
> docs/tools/qemu-img.rst | 6 +-
> Makefile.objs | 2 +-
> include/sysemu/blockdev.h | 10 ++
> blockbitmaps.c | 217 +++++++++++++++++++++++++++++++++++++
> blockdev.c | 184 -------------------------------
> qemu-img.c | 81 +++++++++++++-
> MAINTAINERS | 1 +
> qemu-img-cmds.hx | 4 +-
> tests/qemu-iotests/291 | 143 ++++++++++++++++++++++++
> tests/qemu-iotests/291.out | 56 ++++++++++
> tests/qemu-iotests/group | 1 +
> 11 files changed, 514 insertions(+), 191 deletions(-)
> create mode 100644 blockbitmaps.c
> create mode 100755 tests/qemu-iotests/291
> create mode 100644 tests/qemu-iotests/291.out
>
> --
> 2.26.0
>
