[linux-lvm] lvm-vdo, snapshots and cache

[linux-lvm] lvm-vdo, snapshots and cache

[Gionatan Danti]

Dear all,
as previous vdo utils are gone in RHEL9, VDO volumes must be created via 
lvm-vdo and associated lvm commands.

What is not clear to me is how to combine lvm-vdo with lvmthin (for fast 
CoW snapshots) and/or lvmcache (for SSD caching of an HDD pool). For 
example, with the old /dev/mapper/vdo style volumes (ie: the one created 
via vdo create) one can do something as physical_dev -> vdo -> lvmthin 
-> lvmcache.

How to do the same with lvm-vdo?
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

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Re: [linux-lvm] lvm-vdo, snapshots and cache

[Zdenek Kabelac]

Dne 15. 11. 22 v 18:42 Gionatan Danti napsal(a):
> Dear all,
> as previous vdo utils are gone in RHEL9, VDO volumes must be created via 
> lvm-vdo and associated lvm commands.
> 
> What is not clear to me is how to combine lvm-vdo with lvmthin (for fast CoW 
> snapshots) and/or lvmcache (for SSD caching of an HDD pool). For example, with 
> the old /dev/mapper/vdo style volumes (ie: the one created via vdo create) one 
> can do something as physical_dev -> vdo -> lvmthin -> lvmcache.
> 
> How to do the same with lvm-vdo?


Hi

You could try  'vg' on top of another 'vg' - however I'd not recommend to use 
it this way (and it's unssuported (&unsupportable) by lvm2 in general)

We aim to support multiple VDO LV within a single VDOPOOL - will hopefully 
happen relatively soon  (so it should work like with thin pools)

IMHO I'd not recommend to combine 2 provisioning technologies - it's already 
hard to resolve 'out-of-space' troubles with just one technology...

We will see if the 'VDO thick snapshot' will also go with multiple VDO LV 
support - likely not - but will be added later on.

Caching should be already support - on VDO LV as well as on VDOPOOL LV (both 
work somewhat differently).

Regards

Zdenek

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Re: [linux-lvm] lvm-vdo, snapshots and cache

[Gionatan Danti]

Il 2022-11-15 22:56 Zdenek Kabelac ha scritto:
> You could try  'vg' on top of another 'vg' - however I'd not recommend
> to use it this way (and it's unssuported (&unsupportable) by lvm2 in
> general)

Hi Zdenek,
yeah, I would strongly avoid that outside lab testing.

> IMHO I'd not recommend to combine 2 provisioning technologies - it's
> already hard to resolve 'out-of-space' troubles with just one
> technology...

The issue is that fast snapshots are only provided by dm-thin, meaning 
that lvmthin is almost mandatory for implementing a rolling/continuous 
snapshot scheme. And, on a more basic level, if we can not stack LVM on 
top of a VDO LV (in a supported manner), how can we take a snapshot of 
user data residing on that specific volume?

This seems a clear regression vs a separate VDO device. Am I missing 
something?

> Caching should be already support - on VDO LV as well as on VDOPOOL LV
> (both work somewhat differently).

Good, thanks for confirming. I will do some tests...
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

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Re: [linux-lvm] lvm-vdo, snapshots and cache

[Zdenek Kabelac]

Dne 15. 11. 22 v 23:41 Gionatan Danti napsal(a):
> Il 2022-11-15 22:56 Zdenek Kabelac ha scritto:
>> You could try  'vg' on top of another 'vg' - however I'd not recommend
>> to use it this way (and it's unssuported (&unsupportable) by lvm2 in
>> general)
> 
> Hi Zdenek,
> yeah, I would strongly avoid that outside lab testing.
> 
>> IMHO I'd not recommend to combine 2 provisioning technologies - it's
>> already hard to resolve 'out-of-space' troubles with just one
>> technology...
> 
> The issue is that fast snapshots are only provided by dm-thin, meaning that 
> lvmthin is almost mandatory for implementing a rolling/continuous snapshot 
> scheme. And, on a more basic level, if we can not stack LVM on top of a VDO LV 
> (in a supported manner), how can we take a snapshot of user data residing on 
> that specific volume?

Well - as said - vg on vg is basically equivalent of the original LV on top of 
VDO manager.

But still these fast snapshot do not solve you a problem of double 
out-of-space fault.

> This seems a clear regression vs a separate VDO device. Am I missing something?

It's good for experimenting - but I'd not suggest to use this setup.

Regards

Zdenek

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Re: [linux-lvm] lvm-vdo, snapshots and cache

[Gionatan Danti]

Il 2022-11-16 11:50 Zdenek Kabelac ha scritto:
> Well - as said - vg on vg is basically equivalent of the original LV
> on top of VDO manager.

Hi Zdenek,
it seems clunkier to manage two nested VG/LV if you ask me.

> But still these fast snapshot do not solve you a problem of double
> out-of-space fault.

Yeah, but available space requires constant monitor even when dealing 
with lvmthin.

> It's good for experimenting - but I'd not suggest to use this setup.

In RHEL7 it was specifically supported - see here[1]:

"As a general rule, you should place certain storage layers under VDO 
and others on top of VDO:
Under VDO: DM-Multipath, DM-Crypt, and software RAID (LVM or mdraid).
On top of VDO: LVM cache, LVM snapshots, and LVM Thin Provisioning"

For a practical example: in the past I experimented with a storage stack 
composed of raid -> vdo -> lvmthin -> xfs serving qemu/kvm virtual disk 
files. The lvmthin layer enabled rolling snapshots, and was a key 
feature of the test setup.

Now the only method to replicate that is to have two nested LVM 
instances, one running vdo and one lvmthin - right?

Regards.

[1] 
https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/storage_administration_guide/vdo-qs-requirements

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

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Re: [linux-lvm] lvm-vdo, snapshots and cache

[Zdenek Kabelac]

Dne 16. 11. 22 v 15:38 Gionatan Danti napsal(a):
> Il 2022-11-16 11:50 Zdenek Kabelac ha scritto:
>> Well - as said - vg on vg is basically equivalent of the original LV
>> on top of VDO manager.
> 
> Hi Zdenek,
> it seems clunkier to manage two nested VG/LV if you ask me.

IMHO not different from lvm on top of vdo managed volume.
It should be mostly equal (and in few cases possibly even easier or at least 
more OS friendly)

> 
>> But still these fast snapshot do not solve you a problem of double
>> out-of-space fault.
> 
> Yeah, but available space requires constant monitor even when dealing with 
> lvmthin.
> 
>> It's good for experimenting - but I'd not suggest to use this setup.
> 
> In RHEL7 it was specifically supported - see here[1]:
> 
> "As a general rule, you should place certain storage layers under VDO and 
> others on top of VDO:
> Under VDO: DM-Multipath, DM-Crypt, and software RAID (LVM or mdraid).
> On top of VDO: LVM cache, LVM snapshots, and LVM Thin Provisioning"
> 
> For a practical example: in the past I experimented with a storage stack 
> composed of raid -> vdo -> lvmthin -> xfs serving qemu/kvm virtual disk files. 
> The lvmthin layer enabled rolling snapshots, and was a key feature of the test 
> setup.
> 
> Now the only method to replicate that is to have two nested LVM instances, one 
> running vdo and one lvmthin - right?
> 

Well - it's not difficult 'coding' to enable using 'VDO LV' as thinpool data 
LV, however we currently lack a lot on support for 'VDO LV' recovery alone in 
lvm2 - so until this will get polished - we try to avoid shot ourself to our 
foots with enabling these complicated stacks ATM :)

We also have some ideas for different snapshoting inside VDO - so we will see 
how that will go - it looks more supportable then combining VDO+THIN together.

Regards

Zdenek

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