[linux-lvm] RAID-less parity?

[linux-lvm] RAID-less parity?

[Janne Heß]

Hello everyone,

I'm currently testing dm-integrity and its use with LVM.
For RAID 1,5,6 LVM should just be able to recover the RAID when integrity fails (and the block device returns a read error).
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.
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.
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.

Thanks and regards
Janne

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

[Gionatan Danti]

On 7/13/20 4:34 PM, Janne Heß wrote:
> Hello everyone,
> 
> I'm currently testing dm-integrity and its use with LVM.
> For RAID 1,5,6 LVM should just be able to recover the RAID when integrity fails (and the block device returns a read error).
> 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.
> 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.
> 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.

Hi,
on current LVM implementation, the number of stripes can not exceed the 
number of physical device.

The only possibility I can think of is to partition your disk in 
multiple regions (ie: 3 partitions), adding them as PV to a new VG and 
the creating a RAID4/5 type volume.

Anyway, for good recovery chances you *need* physically different 
devices; otherwise, a catastrophic failure on your single device will 
eat all your data.

Regards.

-- 
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti@assyoma.it - info@assyoma.it
GPG public key ID: FF5F32A8

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

[David Teigland]

On Mon, Jul 13, 2020 at 04:34:52PM +0200, Janne He� 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.

> 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.

Thanks,
Dave

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

[Gionatan Danti]

Il 2020-07-14 18:05 David Teigland ha scritto:
> 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.
> 
> dm-integrity will detect but not fix bit rot.

Hi, I really think error detection has value by itself.
So I would suggest implementing it.

> 
>> 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.

For this feature (parity+linear) I don't see much value: if a single 
physical devices fails hard, all bet of recovery are off. However having 
it can sometime be useful so, if implementing/maintenance cost are low, 
why not?

Thanks.

-- 
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti@assyoma.it - info@assyoma.it
GPG public key ID: FF5F32A8

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

[Janne Heß]

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