[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