Because of Queue Depth reasons, use SSD in Raid 0 mode and not Raid 1 mode
When SATA or SAS SSDs are used as a Cache Device, configure those behind the RAID controller in RAID 0 mode and not RAID 1 mode. This is because, when RAID 1 mode is used, the Queue Depth for the device might reduce substantially.
As an example, the ‘HP Smart Array P244br Controller’ has a default QD of 1011, however when SSDs are in RAID-1 mode, the QD drops to just 10. In RAID 0 mode, and if the SSD is the only device on the RAID controller, the SSD device gets the entire Queue Depth of 1011. Having a higher Queue Depth ensures that the RAID controller doesn’t become the bottleneck.