[linux-lvm] lvmcache with vdo - inconsistent block size

[Gionatan Danti]

Il 2020-09-15 20:34 Zdenek Kabelac ha scritto:
> Dne 14. 09. 20 v 23:44 Gionatan Danti napsal(a):
>> Hi all,
>> I am testing lvmcache with VDO and I have issue with devices block 
>> size.
>> 
>> The big & slow VDO device is on top of a 4-disk MD RAID 10 device 
>> (itself on top of dm-integrity). Over the VDO device I created a 
>> thinpool and a thinvol [1]. When adding the cache device to the volume 
>> group via vgextend, I get an error stating "Devices have inconsistent 
>> logical block sizes (4096 and 512)." [2]
>> 
>> Now, I know why the error shows and what i means. However, I don't 
>> know how to force the cache device to act as a 4k sector device, 
>> and/if this is really required to cache a VDO device.
>> 
>> My current workaround is to set VDO with --emulate512=enabled, but 
>> this can be suboptimal and it is not recommended.
>> 
>> Any idea on what I am doing wrong?
> 
> Hi
> 
> LVM currently does not support mixing devices of different sector sizes 
> within
> a single VG as it brings lot of troubles we have not yet clear vision 
> what
> to do with all of them.

Hi Zdenek, yes, I understand. What surprised me is that lvmvdo *can* be 
combined with caching, and it does not suffer from this issue. Can you 
elaborate on why it works in this case?

> Also this combination of provisioned devices is not advised - since
> you are combining 2 kind of devices on top of each other and it can be
> a big problem
> to solve recovery case.

True.

> On lvm2 side we do not allow to use 'VDO LV' as backend for thin-pool 
> device.

I noticed it. However, from what I can read on RedHat docs, thinpool 
over VDO device should be perfectly fine (the other way around, not so 
much).

> So ATM it's on a user to solve all the possible scenarios that may 
> appear on
> such device stack.
> 
> Zdenek

Thanks.

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Danti Gionatan
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