[linux-lvm] lvmcache with vdo - inconsistent block size
Zdenek Kabelac
zkabelac at redhat.com
Thu Sep 17 19:27:20 UTC 2020
Dne 16. 09. 20 v 0:32 Gionatan Danti napsal(a):
> 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).
>
You've most likely found the bug and this should be likely disable
(and enabled only with some force option).
Problem is, when such device stack is used for XFS - where the 'geometry'
changes, but for some other it's not a big issue (i.e. ext4).
So if you already hit some problem - feel free to open upstream BZ for this issue.
Zdenek
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