Reduce the number of I/O ops when doing COW

Hi all,

here's a patch series that rewrites the copy-on-write code in the qcow2 driver to reduce the number of I/O operations.

This is version v4, please refer to the original e-mail for a complete description:

https://lists.gnu.org/archive/html/qemu-block/2017-05/msg00882.html

Regards,

Berto

v4:
- Patch 6: Initialize the QEMUIOVector with only 1 iovecs [Kevin]
- Patch 7: Initialize the QEMUIOVector with 2 + m->data_qiov->niov [Kevin]
- Patch 7: Rename can_merge_cow() to merge_cow() [Kevin]

v3: https://lists.gnu.org/archive/html/qemu-block/2017-06/msg00472.html
- Patch 4: Over-allocate the read buffer in perform_cow() so both COW sectors are optimally aligned [Eric]

v2: https://lists.gnu.org/archive/html/qemu-block/2017-06/msg00227.html
- Patch 1: Update commit message [Eric]
- Patch 7: Make sure that the number of iovs does not exceed IOV_MAX [Anton]
- Patch 7: Don't add zero-length buffers to the qiov in perform_cow()

v1: https://lists.gnu.org/archive/html/qemu-block/2017-05/msg00882.html
- Initial version

Output of git-backport-diff against v3:

Key:
[----] : patches are identical
[####] : number of functional differences between upstream/downstream patch [down] : patch is downstream-only
The flags [FC] indicate (F)unctional and (C)ontextual differences, respectively

001/7:[----] [--] 'qcow2: Remove unused Error variable in do_perform_cow()' 002/7:[----] [--] 'qcow2: Use unsigned int for both members of Qcow2COWRegion' 003/7:[----] [--] 'qcow2: Make perform_cow() call do_perform_cow() twice' 004/7:[----] [--] 'qcow2: Split do_perform_cow() into _read(), _encrypt() and _write()' 005/7:[----] [--] 'qcow2: Allow reading both COW regions with only one request' 006/7:[0002] [FC] 'qcow2: Pass a QEMUIOVector to do_perform_cow_{read,write}()' 007/7:[0008] [FC] 'qcow2: Merge the writing of the COW regions with the guest data'

Alberto Garcia (7):
qcow2: Remove unused Error variable in do_perform_cow()
qcow2: Use unsigned int for both members of Qcow2COWRegion
qcow2: Make perform_cow() call do_perform_cow() twice
qcow2: Split do_perform_cow() into _read(), _encrypt() and _write()
qcow2: Allow reading both COW regions with only one request
qcow2: Pass a QEMUIOVector to do_perform_cow_{read,write}()
qcow2: Merge the writing of the COW regions with the guest data

 block/qcow2-cluster.c | 201 ++++++++++++++++++++++++++++++++++++++------------
 block/qcow2.c         |  64 +++++++++++++---
 block/qcow2.h         |  11 ++-
 3 files changed, 216 insertions(+), 60 deletions(-)

2.11.0