Re: [RFC] zodcache - auto-start dm-cache devices

[Zdenek Kabelac <zkabelac@redhat.com>]

Dne 18.12.2015 v 21:50 Ian Pilcher napsal(a):
> On 12/18/2015 12:51 PM, Alasdair G Kergon wrote:
>> Did you try stacking LVM on top of itself first?
>
> Honestly, no (although my initial testing of zodcache actually used
> logical volumes as the origin and cache devices FWIW).
>
> This exercise is as much about the "optics" as it is about the actual
> technical merits of the solution.  The goal is provide something that
> (at least on the surface) looks "simple".
>
> With just a couple of obvious enhancements to the mkzc utility (clear
> the metadata region and start the new device), creating a cached device
> would literally take just a single command.
>

Sure - as long as you do not need to solve any 'security/safety' points,
you can do things in a simple way.

But how do you plan to support any power-failures and recoveries ?

When someone pick drive to other system - what is going to happen ?

Are you going to write your own 'lvm2-like' tools to do the job ?

Are you going to maintain this over long time period ?

You really should have started from RFE on lvm2 enhancements to better suite
your needs -  since IMHO there are no SIMPLE things when we talk about devices...

Regards

Zdenek