tablespace to benefit from ssd ?

Hi

I have both hdd and ssd disk on the postgres server. The cluster is
right now created on the hdd only. I am considering using a tablespace
to put some highly used postgres object on the ssd disk. Of course the
ssd is small compared to the hdd, and I need to choose carefully what
objects are stored on that side.

I am wondering what kind of object (indexes, data) would benefit from
ssd. The database primary/foreign keys are highly used and there is
almost no sequencial scan. However the server has a large amount of ram
memory and I suspect all of those indexes are already cached in ram.

I have read that tablespaces introduce overhead of maintenance and
introduce complication for replication. But on the other hand I have
this ssd disk ready for something.

Any recommandation ?

-- 
nicolas paris