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Date: Mon, 27 Jul 2009 10:23:19 +0100
From: Jamie Lokier <jamie@shareable.org>
Subject: Re: [Qemu-devel] [PATCH] Add new block driver for the VDI format
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To: Kevin Wolf <kwolf@redhat.com>
Cc: QEMU Developers <qemu-devel@nongnu.org>, Christoph Hellwig <hch@lst.de>

Kevin Wolf wrote:
> Though I guess that for static images typically not only metadata is
> preallocated, but zeros are written for the whole disk content? Maybe we
> could implement a three-way flag like preallocate=[no,metadata,data] and
> let qemu-img handle the data part (writing zeros is the same for all
> formats and would even work with raw).

Note that you can also preallocate space with posix_fallocate(), which
fills the file with zeros but (sometimes) doesn't take as long as
writing zeros.

Apparently it is almost essential when writing large files in small
pieces on Windows, and on Linux it is supported by the ext4
filesystem, but I haven't checked either claim.

-- Jamie