SCSI

A SCSI command and a parallel SCSI cable are different things. The historical parallel bus is only one interconnect; Linux still uses its SCSI subsystem for several storage paths. Reading a device through that subsystem does not identify the wire protocol. [3]

The standards are layered. SAM describes the architecture, SPC supplies common commands, and device-specific families include SBC for block devices, SSC for sequential-access devices and SMC for media changers. Transport specifications describe how exchanges are carried. Do not infer a disk's operations from its connector or apply disk commands indiscriminately to tape. [1]

An initiator issues commands to logical units through a target. A target can have multiple ports; a port is not necessarily a physical socket. The iSCSI mapping, for example, distinguishes SCSI ports from network portals. [4]

The logical unit (LU) executes I/O commands; the logical unit number (LUN) addresses it within a target. The same numeric LUN on two targets does not establish that they expose the same logical unit or backing storage. An array volume is one possible LU, not the definition of a LUN. [5] [6]

The command descriptor block (CDB) carries the command, distinct from the transferred user data. The response status and any sense information are separate evidence. In iSCSI, autosense is mandatory: error handling should not assume that CHECK CONDITION always requires a subsequent REQUEST SENSE command. [7] [4]

For iSCSI diagnosis, check the Response field before SCSI Status. A nonzero Response means Status is undefined and must be ignored. BUSY and TASK SET FULL do not imply CHECK CONDITION sense data. [4]

Discovery limits also need context. In Linux v6.12, max_luns defaults to 512 for the sequential-scan path; REPORT LUNS processing uses a separate host-adapter max_lun check. The 512 setting is not a universal limit on Linux SCSI volumes. Device quirks and scan configuration affect the path taken. [8]