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(-)
--
2.17.1
