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Date:   Wed, 25 May 2022 02:26:45 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     Qu Wenruo <quwenruo.btrfs@gmx.com>
Cc:     Christoph Hellwig <hch@infradead.org>, dsterba@suse.cz,
        Qu Wenruo <wqu@suse.com>, linux-btrfs@vger.kernel.org
Subject: Re: [PATCH DRAFT] btrfs: RAID56J journal on-disk format draft
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On Wed, May 25, 2022 at 05:13:11PM +0800, Qu Wenruo wrote:
> The problem is, we can have partial write for RAID56, no matter if we
> use NODATACOW or not.
> 
> For example, we have a very typical 3 disks RAID5:
> 
> 	0	32K	64K
> Disk 1  |DDDDDDD|       |
> Disk 2  |ddddddd|ddddddd|
> Disk 3  |PPPPPPP|PPPPPPP|
> 
> 
> D = old data, it's there for a while.
> d = new data, we want to write.

Oh.  I keep forgetting that the striping is entirely on the physіcal
block basis and not logic block basis.  Which makes the whole idea
of btrfs integrated raid5/6 not all that useful compared to just using
mdraid :(