Commit 79c053bd authored by Stefan Hajnoczi's avatar Stefan Hajnoczi Committed by Kevin Wolf
Browse files

block: perform zero-detection during copy-on-read 



Copy-on-Read populates the image file with data read from a backing
image.  In order to avoid bloating the image file when all zeroes are
read we should scan the buffer and perform an optimized zero write
operation.

Signed-off-by: default avatarStefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: default avatarKevin Wolf <kwolf@redhat.com>
parent f08f2dda

block.c

+11 −3
Original line number Diff line number Diff line
@@ -1517,6 +1517,7 @@ static int coroutine_fn bdrv_co_do_copy_on_readv(BlockDriverState *bs, 
     */

    void *bounce_buffer;



    BlockDriver *drv = bs->drv;

    struct iovec iov;

    QEMUIOVector bounce_qiov;

    int64_t cluster_sector_num;
@@ -1537,14 +1538,21 @@ static int coroutine_fn bdrv_co_do_copy_on_readv(BlockDriverState *bs, 
    iov.iov_base = bounce_buffer = qemu_blockalign(bs, iov.iov_len);

    qemu_iovec_init_external(&bounce_qiov, &iov, 1);



    ret = bs->drv->bdrv_co_readv(bs, cluster_sector_num, cluster_nb_sectors,

    ret = drv->bdrv_co_readv(bs, cluster_sector_num, cluster_nb_sectors,

                             &bounce_qiov);

    if (ret < 0) {

        goto err;

    }



    ret = bs->drv->bdrv_co_writev(bs, cluster_sector_num, cluster_nb_sectors,

    if (drv->bdrv_co_write_zeroes &&

        buffer_is_zero(bounce_buffer, iov.iov_len)) {

        ret = drv->bdrv_co_write_zeroes(bs, cluster_sector_num,

                                        cluster_nb_sectors);

    } else {

        ret = drv->bdrv_co_writev(bs, cluster_sector_num, cluster_nb_sectors,

                                  &bounce_qiov);

    }



    if (ret < 0) {

        /* It might be okay to ignore write errors for guest requests.  If this

         * is a deliberate copy-on-read then we don't want to ignore the error.