On 08/14/2018 09:56 PM, Eric Blake wrote:
I was trying to test NBD fleecing by copying subsets of one
file to another, and had the idea to use:
$ export NBD drive to be fleeced on port 10809
$ qemu-img create -f qcow2 copy $size
$ qemu-nbd -f qcow2 -p 10810 copy
$ qemu-img dd -f raw -O raw if=nbd://localhost:10809 of=nbd://localhost:10810 \
skip=$offset seek=$offset count=$((len/cluster)) bs=$cluster
except that seek= wasn't implemented. And in implementing that,
I learned that skip= is broken when combined with count=.
[In the meantime, I had to use:
$ export NBD drive to be fleeced on port 10809
$ modprobe nbd
$ qemu-nbd -c /dev/nbd0 -f raw nbd://localhost:10809
$ qemu-nbd -c /dev/nbd1 -f qcow2 copy
$ dd if=/dev/nbd0 of/dev/nbd1 \
Oops, left out one = on that line.
skip=$offset seek=$offset count=$((len/cluster)) bs=$cluster
And this needs to be Rather, skip=$((offset/cluster))
seek=$((offset/cluster)), unless iflag=skip_bytes oflag=seek_bytes is
also in use.
What's more, it's essential to use conf=fdatasync when scripting this,
otherwise, the dd process can end while the data is still in buffers,
and if 'qemu-nbd -d /dev/nbd1' follows too closely, those buffers are
lost instead of flushed. (I lost the better part of a day figuring out
why things worked when I did it by hand but not when I scripted it,
until finally figuring out that the final flush is mandatory to avoid
data loss).
to get the behavior I needed (basically, create an empty qcow2
destination file, then plug in the guest-visible data based on
the subsets of the disk of my choosing, by reading the block
status/dirty bitmap over NBD). But bouncing through three
NBD client/server pairs just so I can use plain 'dd' instead
of just two pairs with 'qemu-img dd' feels dirty.
]
Eric Blake (2):
qemu-img: Fix dd with skip= and count=
qemu-img: Add dd seek= option
qemu-img.c | 76 ++++++----
tests/qemu-iotests/160 | 15 +-
tests/qemu-iotests/160.out | 344 ++++++++++++++++++++++++++++++++++++++++++++-
3 files changed, 397 insertions(+), 38 deletions(-)
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org
