Re: [PATCH 01/19] btrfs: require at least 4 devices for RAID 6
David Sterba <[email protected]>
| Newsgroups | gmane.linux.ports.ppc.embedded,gmane.linux.kernel,gmane.linux.ports.arm.kernel,gmane.linux.ports.riscv,gmane.linux.kernel.cryptoapi,gmane.comp.file-systems.btrfs,gmane.linux.kernel.cross-arch,gmane.linux.raid |
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| Message-ID | <[email protected]> |
On Thu, May 14, 2026 at 12:57:53PM -0700, H. Peter Anvin wrote: > On May 14, 2026 12:51:59 PM PDT, Goffredo Baroncelli <[email protected]> wrote: > >On 13/05/2026 07.47, Christoph Hellwig wrote: > >> On Tue, May 12, 2026 at 01:42:31PM +0200, David Sterba wrote: > > > >> > >>> The degenerate modes of > >>> raid0, 5, or 6 are explicit as a possible middle step when converting > >>> profiles. We can use a fallback implementation for this case if the > >>> accelerated implementations cannot do it. > >> > >> This is not about a degenerated mode. For a degenerated RAID 6, parity > >> generation uses the RAID 5 XOR routines as the second parity will be > >> missing. This is about generating two parities for a single data disk, > >> which must be explicitly selected. > >> > > > >I think that the David concern is : "what happens for an already > >existing btrfs raid6 3 disks filesystem when the user upgrade the kernel ?" > >(I am thinking when a new BG needs to be allocated)... > > That's what I'm saying – it should invoke the RAID-1 code under the cover (as with 3 disks, D = P = Q.) Thanks, it was not clear to me what you meant. For the two edge cases the code should do simple memcpy for both calculations of parity and recovery.