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Date:   Thu, 28 Nov 2019 11:37:05 +0000
From:   Hubert Tonneau <hubert.tonneau@fullpliant.org>
To:     Goffredo Baroncelli <kreijack@libero.it>
Cc:     linux-btrfs@vger.kernel.org
Subject: Re: Avoiding BRTFS RAID5 write hole
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Goffredo Baroncelli wrote:
>
> You are suggesting to keep the "hot" data in a RAID1 block group, and write/update the RADID5 block group only when the data is "cold" in the RAID1 cache.

Following this view, just adding a 'source data profile' (extent filter) and a 'target data profile' (overules filesystem data profile) option to 'btrfs filesystem defragment' utility would be a good starting point.
It would allow, within a RAID1 btrfs filesystem, to manualy convert some subtree to RAID5, and increase free disk space as a result, so get the best of both worlds.