[Qemu-devel] [PATCH RFC for-3.0-rc3 0/3] qemu-img: Disable copy offloading by default

[Fam Zheng <famz@redhat.com>]

Kevin pointed out that both glibc and kernel provides a slow fallback of
copy_file_range which hurts thin provisioning. This is particularly true for
thin LVs, because host_device driver cannot get allocation info from the
volume, and copy_file_range is called on every sectors, making the dst fully
allocated.

NFS mount points also doesn't support SEEK_DATA well, so the allocation
information is unknown to QEMU.

That leaves only iscsi:// which seems to do what we want so far, but it is a
smaller use case.

Add an option to qemu-img convert, "-C", to enable (attempting) copy offloading
explicitly. And mark it incompatible with "-S" and "-c".

Fam Zheng (3):
  Revert "qemu-img: Document copy offloading implications with -S and
    -c"
  qemu-img: Add -C option for convert with copy offloading
  iotests: Add test for 'qemu-img convert -C' compatibility

 qemu-img-cmds.hx           |  2 +-
 qemu-img.c                 | 21 +++++++++++++++++----
 qemu-img.texi              | 14 +++++++++-----
 tests/qemu-iotests/082     |  8 ++++++++
 tests/qemu-iotests/082.out | 11 +++++++++++
 5 files changed, 46 insertions(+), 10 deletions(-)

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2.17.1