Comment 8 for bug 1375487

Reviewed: https://review.openstack.org/126637
Committed: https://git.openstack.org/cgit/openstack/cinder/commit/?id=c42273fbc1983b146180c82b8a34b0d832a6f431
Submitter: Jenkins
Branch: master

commit c42273fbc1983b146180c82b8a34b0d832a6f431
Author: John Griffith <email address hidden>
Date: Tue Oct 7 11:49:58 2014 -0600

    Make sure device support Direct before setting

    We added '-t none' option to the qemu-img convert operation
    in image_utils.py a while back to accomodate a couple of
    backend devices that didn't flush writes on disconnect.
    (Change: I7a04f683add8c23b9125fe837c4048ccc3ac224d)

    The only problem here is that some backend devices don't
    support Direct mode and raise an exception and fail when
    setting this option.

    This patch adds a simple check using dd to see if the dest
    supports the Direct flag and only sets '-t none' if the device
    does in fact support it.

    Additionally it was brought up that even yet other backends
    are using file devices not blk devices. In their case setting
    Direct will still work, however it's sub-optimal as qemu-convert
    has internal mechanisms to make sure flushing etc are done
    correctly and efficiently for those devices. So to accomodate
    that particular use case I'm also adding a check if blk dev
    that can be used for determining whether to set Direct for the
    qemu-convert process.

    Change-Id: I34127ac373ceadcfb6fc2662628b1a91eb7b0046
    Closes-Bug: 1375487