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Date: Tue, 12 Jan 2010 18:42:20 +0100
From: Kevin Wolf <kwolf@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 1/2] block: flush backing_hd in the right
	place
References: <20100112124923.GA19631@lst.de>
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To: Christoph Hellwig <hch@lst.de>
Cc: qemu-devel@nongnu.org

Am 12.01.2010 13:49, schrieb Christoph Hellwig:
> 
> The backing device is only modified from bdrv_commit.  So instead of
> flushing it every time bdrv_flush is called for the front-end device
> only flush it after we're written data to it in bdrv_commit.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> 
> Index: qemu/block.c
> ===================================================================
> --- qemu.orig/block.c	2010-01-12 11:34:35.549024986 +0100
> +++ qemu/block.c	2010-01-12 11:43:28.965006129 +0100
> @@ -623,6 +623,12 @@ int bdrv_commit(BlockDriverState *bs)
>      if (drv->bdrv_make_empty)
>  	return drv->bdrv_make_empty(bs);
>  
> +    /*
> +     * Make sure all data we wrote to the backing device is actually
> +     * stable on disk.
> +     */
> +    if (bs->backing_hd)
> +        bdrv_flush(bs->backing_hd);
>      return 0;
>  }

Format drivers with a bdrv_make_empty return before the flush, so it
won't work for qcow1. Looks good otherwise.

If it has done a bdrv_make_empty we might also want to flush bs?

Kevin