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Date: Mon, 25 Aug 2014 14:44:52 +0100
From: "Richard W.M. Jones" <rjones@redhat.com>
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In-Reply-To: <20140825051830.GC25054@G08FNSTD100614.fnst.cn.fujitsu.com>
Subject: Re: [Qemu-devel] [PATCH v12 0/6] qcow2,
 raw: add preallocation=full and preallocation=falloc
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To: Hu Tao <hutao@cn.fujitsu.com>
Cc: Kevin Wolf <kwolf@redhat.com>, qemu-devel@nongnu.org, Max Reitz <mreitz@redhat.com>, Stefan Hajnoczi <stefanha@redhat.com>, Yasunori Goto <y-goto@jp.fujitsu.com>

On Mon, Aug 25, 2014 at 01:18:30PM +0800, Hu Tao wrote:
> What if user cares about time(writing zeroes or non-zeroes is
> time-consuming) and wants falloc only sometimes? I think this is the
> main difference between preallocation=falloc and preallocation=full.

Also posix_fallocate in glibc falls back to writing zeroes when the
kernel/VFS doesn't support a true fallocate.  So I'm afraid your patch
5/6 has a slow path, at least on common Linux distros.

Rich.

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