Note: This series has been rebased onto the image streaming series which is likely to get merged first. There are no significant changes to this series since v2.
This series adds an interface for efficient writes when data contains all zeros. It also takes advantage of this new interface by extending the copy-on-read feature to perform zero-detection. The details of efficient zero representations depend on the image format. This series includes a patch for the QED image format to write special "zero clusters" that keep the image file compact. In the future qcow2v3 could also support an efficient zero representation. The new BlockDriver interface is called .bdrv_co_write_zeroes() and is optional. If the interface is not implemented by a BlockDriver then a regular .bdrv_co_writev() operation will be performed. The public interface is called bdrv_co_write_zeroes() and can be tested via the new qemu-io write -z option. Copy-on-read is extended to detect zeroes and invoke the .bdrv_co_write_zeroes() interface when possible. As a result we avoid bloating the image file if the backing file contains zeroes. My motivation for this feature is efficient image streaming. The destination file must stay compact even when copying zeroes from the source file. We now only do zero detection for copy-on-read requests, whereas previous revisions of this patch series scanned all write requests for zeroes. The old behavior wasted CPU cycles in most cases but we could add a feature to explicitly scan guest writes for zeroes in the future, if desired. v3: * Rebased on image streaming series v2: * Introduce .bdrv_co_write_zeroes() [Kevin] * Perform zero detection only on copy-on-read requests [Kevin] Stefan Hajnoczi (6): cutils: extract buffer_is_zero() from qemu-img.c block: add .bdrv_co_write_zeroes() interface block: perform zero-detection during copy-on-read qed: replace is_write with flags field qed: add .bdrv_co_write_zeroes() support qemu-io: add write -z option for bdrv_co_write_zeroes block.c | 67 ++++++++++++++++++++++++++---- block.h | 7 +++ block/qed.c | 125 ++++++++++++++++++++++++++++++++++++++++++++++++++------- block/qed.h | 7 +++- block_int.h | 8 ++++ cutils.c | 34 +++++++++++++++ qemu-common.h | 2 + qemu-img.c | 46 +++------------------ qemu-io.c | 77 +++++++++++++++++++++++++++++++---- trace-events | 3 +- 10 files changed, 303 insertions(+), 73 deletions(-) -- 1.7.8.3
