What risk(s) are there in creating a 6 GB logical volume with the following underlying configuration?:
one 2 GB RAID 1 with 2 disks housing one physical volume
one 4 GB RAID 1 with 2 disks housing one physical volume
if either of the PVs gets corrupted, your LV will also, but that is not a likely occurrence when you have raid 1, because it would take both disks to go bad. i am not sure these days we have disks that small, or are you using logical disks? in any case, it is quite a robust setup otherwise. everything has risks and you can really only compare with an alternative configuration to say more.
There is very low risk involved, but the risk is mathematically higher than if you had a single physical volume. For example, if each disk has a 0.05% chance of failure per week (works out to about 2.5% per year, which is a realistic estimate), then the chance for both drives to fail in the same week is 0.000025%, or one in 4 million.
But when you expand that to your drive configuration, then you have two chances for double-drive failure, doubling your failure rate to about one in 2 million per week. It's still vanishingly small, but the same article that I cited for failure rates indicates that drives often fail in cascades, dramatically increasing the chance for multi-drive failure over the base rate you would expect. The odds are still not that bad, but that depends on your use case and business security needs to determine what risks are acceptable.
Whatever the risks are for two drives failing simultaneously, spanning a logical volume across two physical pairs doubles that number.