Hi Norman
There is no fixed percentage for that. If you mix slow and fast OSD in a PG, the overall
performance of the pool will be affected by the slower devices. This is the same reason as
you will not mix SSD and HDD OSD in a pool. If the performance difference between OSD is
small, it will be OK to mix. It is also the same recommendation if you mix different
performances of any x86 servers.
Best regards,
Aaron