LVM: Create thin pools of adequate size

Thin pools in LVM are quite different from volume groups or logical
volumes and their differences must be taken into account when providing
thin LVM support in Cinder.

When you create a thin pool, LVM actually creates 4 block devices.  You
can see this after thin pool creation with the following command:

    $ dmsetup ls

    volumes--1-volumes--1--pool       (253:4)
    volumes--1-volumes--1--pool-tpool (253:3)
    volumes--1-volumes--1--pool_tdata (253:2)
    volumes--1-volumes--1--pool_tmeta (253:1)

In the above command, a thin pool named 'volumes-1-pool' was created in
the 'volumes-1' volume group.  Despite this, the 'lvs' command will only
show one logical volume for the thin pool, which can be misleading if
you aren't aware of how thin pools are implemented.

When you create a thin pool, you specify on the command line a size for
the pool.  LVM will interpret this size as the amount of space requested
to store data blocks only.  In order to allow volume sharing and
snapshots, some amount of metadata must be reserved in addition to the
data request.  This amount is calculated by LVM internally and varies
depending on volume size and chunksize.  This is why one cannot simply
allocate 100% of a volume group to a thin pool - there must be some
remaining space for metadata or you will not be able to create volumes
and snapshots that are pool-backed.

This patch allocates 95% of a volume group's free space to the thin
pool.  By doing this, we allow LVM to successfully allocate a region for
metadata.  Additionally, any free space remaining will by dynamically
used by either data or metadata if capacity should become scarce.

The 95/5 split seems like a sane default.  This split can easily (and
probably should) be made user-configurable in the future if the user
expects an abnormal amount of volume sharing.

Change-Id: Id461445780c1574db316ede0c0194736e71640d0
Closes-Bug: #1245909
Merge "LVM: Create thin pools of adequate size"