What would a good OSD node hardware configuration look like?

[Dennis Jacobfeuerborn <dennisml@conversis.de>]

Hi,
I'm thinking about building a ceph cluster and I'm wondering what a good
configuration would look like for 4-8 (and maybe more) 2HU 8-disk or 3HU
16-disk systems.
Would it make sense to make each disk an individual OSD or should I perhaps
create several raid-0 and create OSDs from those?
Also what is the best setup for the journal? If I understand it correctly
then each OSD needs its own journal and that should be a separate disk but
that would be quite wasteful it seems. Would it make sense to put in two
small SSD disks in a raid-1 configuration and create a filesystem for each
OSD journal on it?
How does the number of OSDs/Nodes affect the performance of say a single dd
operation? Will blocks be distributed over the cluster and written/read in
parallel or does the number only improve concurrency rather than benefit
single threaded workloads?

Regards,
  Dennis