Ceph

Questions such as the following are asked frequently on the mailing list and IRC:
- What happens if the journal is lost (the disk on which the journal is written to fails)
- How many OSD journals should you put on another disk
- What kind of performance increase should be expected by putting the journal on another disk

It seems recommended or at least implied that it is better to put the journals on different disks.
We should provide documentation to explain why and what are the possible drawbacks/consequences.

Ref:
http://ceph.com/docs/next/install/hardware-recommendations/#solid-state-drives
http://ceph.com/docs/next/rados/configuration/osd-config-ref/
http://ceph.com/community/ceph-performance-part-2-write-throughput-without-ssd-journals/
http://comments.gmane.org/gmane.comp.file-systems.ceph.user/4267
http://comments.gmane.org/gmane.comp.file-systems.ceph.user/3809