[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

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Danti Gionatan
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