cache support

[Paul B. Henson <henson@acm.org>]

> From: Zdenek Kabelac
> Sent: Wednesday, February 05, 2014 1:30 PM
>
> lvm2 is not supporting caching of PVs - that's the layer below the lvm2.
Your
> proposed idea would be hard to efficiently implement.

Hmm, yes, I see what you mean. I guess to go that way I'd have to use
dm-cache directly or reconsider bcache to create a cached device to feed to
lvm as a PV.

> We may consider the option to use a single cache pool for multiple single
> linear LVs - since in this case we might be able to resolve tricky virtual
> mapping.

It would probably work out for my use case if the PV itself wasn't cached
but the entire SSD cache device could be shared amongst all of the LV's. I
really don't want to split the cache into separate pieces per LV though,
that's just not going to scale.

Thanks.