In all Cisco HX hybrid configurations 1 separate SSD per node is used for houskeeping purposes (SDS logs).
In all Cisco HX hybrid and all-flash configurations 1 separate SSD per node is used for caching purposes. The other disks (SSD or HDD) in the node are used for persistent storage of data.
In a hybrid scenario, the caching SSD is primarily used for both read and write caching. However, data written to SSD is only destaged when needed. This means that current data stays available on the SSD layer as long as possible so that reads and writes are fast.
Distributed Read Cache: All SSD caching drives within the HyperFlex storage cluster form one big caching resource pool. This means that all storage nodes can access the entire distributed caching layer to read data.
In an all-flash scenario, the caching SSD (SAS) is primarily used for write caching. Reads are always accessed directly from the capacity SSD (SATA) layer, so a read cache is not required.