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Date: Tue, 28 Jul 2009 12:07:39 +0530
From: Amit Shah <amit.shah@redhat.com>
Subject: Re: [Qemu-devel] [PATCH] Add new block driver for the VDI format
	(use aio)
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To: Jamie Lokier <jamie@shareable.org>
Cc: Kevin Wolf <kwolf@redhat.com>, QEMU Developers <qemu-devel@nongnu.org>, Christoph Hellwig <hch@lst.de>

On (Mon) Jul 27 2009 [10:23:19], Jamie Lokier wrote:
> 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.

It won't take as long as writing zeroes if the filesystem underneath has
support for fallocate(). ext4, btrfs, xfs have support for fallocate().

> 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.

I did some comparisons:

http://log.amitshah.net/2009/03/comparison-of-file-systems-and-speeding.html

http://log.amitshah.net/2009/04/re-comparing-file-systems.html

		Amit