Backplane

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The board the disks plug into, on the path between them and the HBA or the controller. A passive backplane only routes the SATA or SAS signals from each disk bay to a connector; an active one carries a SAS expander, a switch that lets one cable reach many disks. Either way it stores nothing and computes no RAID.

In a server or a disk shelf the physical disks do not cable to the controller one by one. They slide into bays, and the bays are connectors on a board at the back of the cage: the backplane. Its job is electrical, to carry each disk's signal lines to a connector the HBA or the RAID controller cables to, and to bring power and the bay's status lights.

Two kinds exist, and the difference is whether anything on the board processes commands:

• Passive. Connectors and traces, one lane per bay. A cable from the controller carries four lanes, so a passive backplane serves four bays per connector, and a larger cage needs more cables.
• Active. The board carries a SAS expander, a switch between the controller's lanes and the bays. One four-lane cable then reaches every disk in the cage, and the expander routes each command to its target. The expander only forwards commands; it is not a raid engine, and it keeps no data.

NVMe disks are the case that steps around it: they speak PCIe, and in the simplest wiring they reach the bus without a SAS backplane in between (physical disks).

Why name it at all, if it computes nothing? Because it is a failure domain. A backplane that dies takes every disk on it off the path at once, however the disks are laid out. The practice that follows is to spread the members of one span across backplanes where the cage allows it, so that no single board holds both copies of a mirror or two members of a parity stripe.