cache support

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

> From: Brassow Jonathan
> Sent: Monday, March 17, 2014 8:56 AM
>
> Yes, that is basically the idea.  However, converting the thinpool is a
little
> more tricky.  You already are using the fast device for the thinpool
metadata
> device (which seems awfully large from your example).  When you cache the
> thinpool, I think you just want to cache the data section.
> # lvconvert --type cache --cachepool vg/cachepool vg/thinpool_tdata

Ah right; in your original example you had connected the cache to the
thinpool data lv before creating the actual thinpool, when I tried to
convert that to adding the cache after the fact I didn't take that into
consideration.

In your original example, it said to size the metadata device to
approximately 1/1000 of the data lv size, so for a 4 TB data lv that would
actually be almost 4 GB? The estimation utility says about 2.4 GB:

# thin_metadata_size -b 64k -s 4t -m 100000 -u g
thin_metadata_size - 2.41 gigabytes estimated metadata area size

Evidently online metadata pool resize is supported now, so I guess I don't
have to extremely overestimate from the start, as long as I keep an eye on
it I can always top it off before it runs out.

Thanks.