Why are eight Ceph journals created on an SSD drive
We partition the SSD devices intended for the Ceph Journal like this for performance.
We create eight Journal devices from a single physical disk device as the Ceph Journal does not need much physical disk capacity for operation. The Ceph Journal device needs to be on a SSD based Device or RAID array that has a high IOPS potential.
We chose to allow multiple OSD's to use the same physical SSD device as concurrent operation would allow for better use of the IOPS available from the SSD devices when multiple OSD's use it for their Journal Device.
We allow up to eight OSD's to use a SSD specific Physical SSD array or disk as an OSD Journal device.
We found that if there are more than 8 I/O streams used for Journal that most SSD's used as single disk or as RAID1 begin to quickly have diminishing performance for IOPS.