[linux-lvm] Why does thinpool take 2*poolmetadatasize space?

[Alasdair G Kergon]

On Tue, Oct 20, 2020 at 12:33:41PM -0400, Scott Moser wrote:
> When I create an lvmthinpool with size S and poolmetadatasize P,
> it reduces the available freespace by S+2P. I expected that to
> be S+P. Where did the extra poolmetadatasize get used?

So that you have sufficient disk space to store a second copy of it
if you need to perform any work on the metadata in future.

Recovery operations, for example, create a recovered copy in that
'spare' space then swap to use it leaving the space used by the
original metadata as the new 'spare'.

Alasdair