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Date: Mon, 18 May 2026 07:12:07 +0200
From: Christoph Hellwig <hch@lst.de>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Christoph Hellwig <hch@lst.de>, kreijack@inwind.it,
	David Sterba <dsterba@suse.cz>,
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	linux-raid@vger.kernel.org
Subject: Re: [PATCH 01/19] btrfs: require at least 4 devices for RAID 6
Message-ID: <20260518051207.GB9374@lst.de>
References: <20260512052230.2947683-1-hch@lst.de> <20260512052230.2947683-2-hch@lst.de> <20260512114231.GG2558453@suse.cz> <20260513054742.GA1018@lst.de> <0a8d1ff4-f5a2-49e9-aa45-d25dbe4ded40@libero.it> <20260515043705.GA3855@lst.de> <34C16854-1065-4542-8836-DDED58EC1844@zytor.com>
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On Fri, May 15, 2026 at 12:59:34PM -0700, H. Peter Anvin wrote:
> I don't think this is a good idea. Error out; it is the btrfs maintainers' job to ensure user data isn't lost. 
> 
> The RAID-6 code has *never* supported only 3 units, and if it ever worked for *any* of the implementations it was purely by accident. Speaking as the original author I should know; this was deliberate as in some cases the degenerate case (3) would have required extra trays in the code to no user benefit. 
> 
> I would not be surprised if the kernel crashed or corrupted the page cache in that case.

It does, that's why I wanted to exclude it.  Anyway, for the about to be
resent version I'll drop this btrfs patch over the stated objection and
will otherwise not change anything.  This means the (IMHO hypothetical)
users of this configuration will get a WARN_ON_ONCE triggered, but
otherwise keep working (or rather not working) as before.