> there are three pool in ceph cluster, e.g, t1, t2 t3, and t2 is a cache of t1.
> 1, we can set t1 as a cache of t2 too, then t1 is the cache of t2, t2 also is the cache of t1,a loop of cache generated;
> 2, we can also set t3 as a cache of t2 ort t1, then a three level cache tier generated, t3 is the cache of t2, t2 is the cache of t1.
> cache loop, and three level cache are weird, also maybe reduce the IO performance, so it'd be better to avoid complicated tier like them.
we don't support multiple tiering, so we'd better fail a "ceph osd tier add" command which intent to add a tier for another tier.