[linux-lvm] LVM cache/dm-cache questions.

[lejeczek]

On 26/08/16 15:45, Ondrej Kozina wrote:
> On 08/26/2016 04:01 PM, lejeczek wrote:
>> whatever you might call it, it works, luks encrypting,
>> opening & mounting @boot - so I only wonder (which was my
>> question) why not cache pool LVs. Is it not supported...
>> would be great if a developer sees this question, I'm not
>> sure jut yet about filing a bug report.
>
> In general LVM2 doesn't auto-activate or interpret unknown 
> device types. LUKS header is considered unknown from LVM2 
> perspective. Simply put LVM2 doesn't understand LUKS 
> header data. Not sure what you tried to do with cache pool 
> LV, but in my opinion any effort to encrypt (live or 
> detached) cache pool LV may end with severe data 
> corruption...
>
> As of now I think you have in general two options:
>
> a) encrypt both PVs because obviously if you only encrypt 
> the origin PV you end up with decrypted plaintext data 
> stored in cache pool. Probably this is the exact scenario 
> you were about to avoid?
>
> Unfortunately a) is suboptimal with regard to performance 
> since you'd perform the encryption of data blocks twice.
>
> Option b): encrypt the top level LV (the one constructed 
> from both cache and origin LV). This way ciphertext would 
> be stored twice in cache PV and origin PV but the 
> encryption would be performed only once.
>
gee, guys, thanks Ondrej,
this I was saying from the beginning did not work - option b 
- does not work. I can Not encrypt top level cache pool LV.
It does work with any other LV I have, but cache pool fails 
(like I said earlier) with:

Command failed with code 22.

And me speculating on my own - whether it is a bug or just 
limitation of implementation (Centos 7.2, 
lvm2-2.02.130-5.el7_2.4.x86_64, 
cryptsetup-1.6.7-1.el7.x86_64)- I thought instead I should 
seek clarification.

> Regards
> Ondrej
>
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