Hello everyone! The given feature discards blocks with copy-on-read operation while the streaming process runs. Adding the 'discard' argument to the QMP block-stream command allows dropping a block in the backing chain after it has been copied to the active layer. That will elude the block duplication in the intermediate backing file. It saves the disk space while external snapshots are being merged. The method involves the filter insertion above the active layer to allow write operation in the backing chain. The method is similar to that in the 'commit active' command (mirror.c). The permission to write into an inactive layer can not be obtained due to the existing child permission mechanism. There is a commented up hack in the callback function bdrv_stream_top_pwritev() in block/stream.c that redirects write operations below the filter node. Being uncommented, it enables writing into the inactive layer and passing all the iotests in the 030 file. Otherwise, no WRITE permission is granted after the filter insertion above the target node. Any suggestions to resolve that issue will be appreciated.
The suggestions of Dr. David Alan Gilbert and Alberto Garcia after their first review have been applied. Sincerely, Andrey Shinkevich (5): Discard blocks while copy-on-read The discard flag for block stream operation iotests: allow resume_drive by node name iotests: prepare 030 for graph change iotests: 030 with block-stream discard block/stream.c | 429 ++++++++++++++++++++++++++++++++++++++++-- blockdev.c | 8 +- hmp-commands.hx | 4 +- hmp.c | 4 +- include/block/block_int.h | 2 +- qapi/block-core.json | 5 +- tests/qemu-iotests/030 | 163 +++++++++++----- tests/qemu-iotests/030.out | 4 +- tests/qemu-iotests/iotests.py | 9 +- 9 files changed, 558 insertions(+), 70 deletions(-) -- 1.8.3.1
