One of our customers was setting up box for MySQL LVM backups. The box had 2 RAID volumes (internal drives and enclosure) with database being stored on enclosure drives and internal drives used pretty much for OS and logs (database grew too large and so was moved to enclosure completely).
As database was large and active I thought it would make sense to have LVM undo space located on the internal drives so LVM and backup has less backup on the running system.
As the system was configured creating snapshot volume with 100G of undo space failed complaining there is not enough space available on the volume….
The problem was internal and external volumes were configured as different “volume groups”. Volume groups in LVM live completely isolated life and you can’t use space from one volume group for snapshot for another.
What should have done instead is creating single volume group and having RAID volumes as physical volumes.
If you also have done such mistake there is vgmerge command which allows to merge volume groups together without data loss. (Though I took backup anyway just to be on the safe side)
P.S That would be interested to benchmark one day how LVM performs in shapshot mode in case undo space is stored on the same device vs different device.