See the lvmcache(7) manpage, which I think may be what the earlier poster was thinking of. It isn't an asymmetric RAID mode, but a tiered caching scheme where you can, for example, put a faster and smaller enterprise SSD in front of a larger and slower bulk store. So you can have a large bulk volume but the recently/frequently used blocks get the performance of the fast cache volume.

I set it up in the past with an mdadm RAID1 array over SSDs as a caching layer in front of another mdadm array over HDDs. It performed quite well in a developer/compute workstation environment.