As you plan a storage unit configuration, it is important to understand basic concepts about the construction of a server and the relationship between a storage unit and the attached host systems.
The storage unit Fibre Channel adapter is configured for one of two connection topologies: point-to-point and Fibre Channel arbitrated loop. The point-to-point topology supports the SCSI to Fibre Channel protocol (SCSI-FCP) or the FICON protocol. The Fibre Channel arbitrated loop (FCAL) topology supports the SCSI-FCP communications protocol. An storage unit Fibre Channel host adapter supports both SCSI-FCP and FICON protocols.
The host adapter attaches the storage unit to a host system. You can install the adapters to support any combination of ESCON, SCSI, or Fibre Channel connections. The ESCON and SCSI adapters provide two ports (interfaces). The Fibre Channel adapter provides a single port. All ports on a given host adapter must have the same upper layer protocol (ULP) (for example, FICON or SCSI). This requirement allows each port to be configured with an independent ULP.
Using the common-parts interconnect capability, the host adapter can access either of two clusters that are installed in independent bays. The clusters use a symmetric multiprocessor to coordinate data transfer and perform advanced functions between the host adapters and device adapters. Each cluster includes cache and nonvolatile storage.
Cache stores frequently accessed data and makes it quickly available. Nonvolatile storage retains data even with the power off. Nonvolatile storage together with cache storage improves performance of write operations and provides data integrity in the event of an unplanned loss of power to the cluster.
The DS8700 contains solid state drives (SSDs) connected using Fibre Channel adapters. The DS8800 contains large capacity Serial attach SCSI (SAS) drives connected using Fibre Channel adapters.
Arrays are composed of a group of DDMs that store data within the storage unit. DDMs are installed in pairs, with each pair attached to the same loop. Within a DDM eight pack, for example, all the disk drives are the same capacity and speed.