Re: [linux-lvm] RAID-less parity?

["Janne Heß" <janne+lvm@hess.ooo>]

Hi,
Am Dienstag, 14. Juli 2020 18:05 CEST, schrieb David Teigland <teigland@redhat.com>: 
 
> On Mon, Jul 13, 2020 at 04:34:52PM +0200, Janne Heß wrote:
> > However some of my systems are single-disk systems. For those, RAIDs are
> > not possible so I was thinking if LVM has some support for single-PV
> > setups with parity on the same PV.
> 
> Hi,
> 
> We didn't include integrity+linear because we didn't see much value in it,
> since it would require going to backups in case of an integrity failure.
> 
> > While this is probably a niche feature right now, I can see other
> > projects like Stratis benefit from the integrity checks provided by
> > dm-integrity (and dm-crypt in the future) to detect (and thanks to
> > parity fix) bit rots.
> 
> dm-integrity will detect but not fix bit rot.

This is where I see a benefit from having some parity on the LVM level.
With dm-integrity spotting bit-rots and reporting errors to LVM, LVM
can recover the blocks with parity information.
This recovery procedure would give more feature parity to ZFS
which I'm currently migrating from for a LVM-based solution.

> 
> > So is there any way to get a parity setup with the current state of LVM?
> > I was thinking of a RAID-4 with one PV but the current tooling (more
> > specifically lvcreate) doesn't let me do that.
> 
> It's not currently possible.  We had an implementation for this we could
> bring back if there is enough interest, so we'd like to hear more about
> how this would be useful for you and others.

I think this would be beneficial for a lot of users since it allows spotting
and correcting bit-rots which is currently not possible other than using
ZFS or RAID as far as I know.
> 
> Thanks,
> Dave
> 
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> 
 
 Thanks,
Janne