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Date: Fri, 20 Dec 2013 11:10:30 +0100
From: Stefan Hajnoczi <stefanha@gmail.com>
Message-ID: <20131220101030.GD27021@stefanha-thinkpad.redhat.com>
References: <cover.1387419339.git.hutao@cn.fujitsu.com>
	<521cd9f1c99d5912c7438b1714fe4d0de758344e.1387419339.git.hutao@cn.fujitsu.com>
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In-Reply-To: <521cd9f1c99d5912c7438b1714fe4d0de758344e.1387419339.git.hutao@cn.fujitsu.com>
Subject: Re: [Qemu-devel] [RFC PATCH v3 2/6] block: add
 BlockDriver.bdrv_preallocate.
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To: Hu Tao <hutao@cn.fujitsu.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Peter Lieven <pl@kamp.de>, Fam Zheng <famz@redhat.com>, qemu-devel@nongnu.org

On Thu, Dec 19, 2013 at 10:27:37AM +0800, Hu Tao wrote:
> diff --git a/block.c b/block.c
> index 64e7d22..b901587 100644
> --- a/block.c
> +++ b/block.c
> @@ -3216,6 +3216,19 @@ bool bdrv_can_write_zeroes_with_unmap(BlockDriverState *bs)
>      return false;
>  }
>  
> +int bdrv_preallocate(BlockDriverState *bs, int64_t offset, int64_t length)
> +{
> +    if (bs->backing_hd) {
> +        return -ENOTSUP;
> +    }

Depending on the image file format it may be possible to preallocate
metadata while using a backing file.  Why prevent this?