RE: [PATCH] btrfs-progs: mkfs: Enable -d dup for single device

[Zhao Lei <zhaolei@cn.fujitsu.com>]

Hi, David Sterba

Thanks for review.

> -----Original Message-----
> From: David Sterba [mailto:dsterba@suse.cz]
> Sent: Tuesday, October 13, 2015 7:29 PM
> To: Zhao Lei <zhaolei@cn.fujitsu.com>
> Cc: linux-btrfs@vger.kernel.org; clm@fb.com
> Subject: Re: [PATCH] btrfs-progs: mkfs: Enable -d dup for single device
> 
> On Tue, Oct 13, 2015 at 06:29:41PM +0800, Zhao Lei wrote:
> > Current code don't support dup profile in single device, except it is
> > in mixed mode, because following reason:
> > 1: In some ssd with deduplication function, it have no effect.
> > 2: For a physical device, it the entire disk broken, -d dup can
> >    not help.
> > 3: Half performance comparing with single profile.
> > 4: We have a workaround: Create multi-partition in single device,
> >    and btefs will treat them as multi device.
> 
> While the above makes sense is true, I'm not sure that DUP was disabled for
> these reasons. I'm sure that I read a comment from Chris that dup for data is
> intentionally disabled because this would lead to corruption, the code for DUP
> for metadata would not work for data. And I can't find the comment, but the
> doubt is there. So unless I find it or get otherwise convicend that it's ok, I won't
> merge the patch. I hope you understand that.
> 
> What I remember from the comment is that "it's slightly offset that would lead
> to corruption".

Before this patch, I had done git blame to search why the condition was added,
and hadn't found the exact reason.

I will queue xfstests(btrfs/generic) at this profile with all mount option
for multi-times, to check is something wrong with this.

Thanks
Zhaolei