Canisters

Canisters are hardware units that are subcomponents of enclosures.

The system has two types of canisters: node canisters and expansion canisters. A node canister provides host interfaces, management interfaces, and SAS interfaces to the control enclosure. A node canister has the cache memory, the internal drives to store software and logs, and the processing power to run the system's virtualizing and management software. A node canister also contains a battery that helps to protect the system against data loss if a power outage occurs. An expansion canister provides the serial-attached SCSI (SAS) connectivity to the drives in an expansion enclosure. Each enclosure contains a pair of canisters to provide redundancy. An expansion canister does not contain a battery. The left canister is identified as Canister 1. The right canister is identified as Canister 2.

Each node canister in a system contains a battery. If the power to a node canister fails, the node can write its configuration state and cache state to its internal drive using the power provided by the canister battery.

When charged, a canister battery holds enough charge to save the data in the node twice in quick succession. If there is not enough charge in the battery for a node to save its internal state, the node does not start and a critical node error is reported. If battery power drops below the required level while a node is running, it is no longer protected by the battery. The node then safely enters service state while the canister battery recharges.