When would I create a concatenated metadevice?
To expand the capacity of an existing data set, such as a file system.
Concatenation is good for small random I/O and for even I/O distribution.
What are the limitations to concatenation?
Practically speaking, none. You must use a concatenation to encapsulate root (/), swap, /usr, /opt, or /var when mirroring these file systems.
How large can a concatenated metadevice be?
Up to one Terabyte.