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Date: Tue, 31 Jan 2017 14:38:01 +0000
From: Stefan Hajnoczi <stefanha@redhat.com>
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In-Reply-To: <1482503344-6424-9-git-send-email-vsementsov@virtuozzo.com>
Subject: Re: [Qemu-devel] [PATCH 08/21] backup: skip unallocated clusters
 for full mode
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To: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Cc: qemu-block@nongnu.org, qemu-devel@nongnu.org, kwolf@redhat.com, mreitz@redhat.com, jsnow@redhat.com, famz@redhat.com, den@openvz.org, pbonzini@redhat.com, jcody@redhat.com


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On Fri, Dec 23, 2016 at 05:28:51PM +0300, Vladimir Sementsov-Ogievskiy wrote:
> +            if (is_top || bdrv_has_zero_init(blk_bs(job->target))) {

  /*
   * Returns 1 if newly created images are guaranteed to contain
   * only zeros, 0 otherwise.
   */
  int (*bdrv_has_zero_init)(BlockDriverState *bs);

This function does not cover arbitrary images opened with bdrv_open(),
only those created with bdrv_img_create() immediately before opening.

This means that the check is only valid with backup->mode !=
NEW_IMAGE_MODE_EXISTING.

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