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Date: Thu, 4 Sep 2014 14:52:57 +0200
From: Kevin Wolf <kwolf@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v13 5/6] raw-posix: Add full preallocation
	option
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To: "Richard W.M. Jones" <rjones@redhat.com>
Cc: Fam Zheng <famz@redhat.com>, Hu Tao <hutao@cn.fujitsu.com>, qemu-devel@nongnu.org, Max Reitz <mreitz@redhat.com>, Stefan Hajnoczi <stefanha@redhat.com>, Yasunori Goto <y-goto@jp.fujitsu.com>

Am 04.09.2014 um 14:45 hat Richard W.M. Jones geschrieben:
> On Thu, Sep 04, 2014 at 02:35:22PM +0200, Kevin Wolf wrote:
> > Please change the code to always write zeros for FULL,
> 
> How is this useful for anyone?  You don't know if the underlying SAN
> is going to detect these zeroes or combine these blocks together.
> It's just slow for no reason.

It's slow for the reason that the user has requested it. Do you doubt
that users can know what their backend is doing? Why are you insisting
on providing only the functionality that you personally need?

I doubt it's a way to make many users happier, but if you insist, we can
leave full preallocation unsupported on raw-posix and allow only
metadata preallocation (which should still be fallocate() without a
zero-write fallback, so that you can rely on it being fast). But call it
by its name and don't say "full" when you only implement "metadata".

Kevin