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Date: Sun, 16 Aug 2009 21:28:15 +0100
From: Jamie Lokier <jamie@shareable.org>
Subject: Re: [Qemu-devel] [PATCH] qcow2: Metadata preallocation
Message-ID: <20090816202815.GA7162@shareable.org>
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To: Filip Navara <filip.navara@gmail.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Avi Kivity <avi@redhat.com>, qemu-devel@nongnu.org

Jamie Lokier wrote:
> Filip Navara wrote:
> > FILE_ATTRIBUTE_SPARSE_FILE? You can't actually set it when
> > opening/creating the file, a separate call to
> > DeviceIoControl/FSCTL_SET_SPARSE is needed.
> 
> I see that you increase the file size by writing zeros to the end.
> 
> Can't you use the Windows equivalent of unix ftruncate() to extend the
> file instead, after FSCTL_SET_SPARSE?

Specifically: 

    SetEndOfFile
	Use this after SetFilePointer to change the length of a file
	or stream.  If used on a sparse file or stream, increasing
	the length creates a sparse region.

-- Jamie