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BS ISO IEC 14776 — Small Computer System Interface SCSI Standard Series (SAS Protocol Layer and Block Commands)
Information Technology — Small Computer System Interface (SCSI): SAS Protocol Layer — 2 and SCSI Block Commands — 3
💾 BS ISO/IEC 14776 is the Small Computer System Interface (SCSI) standard series jointly developed by ISO and IEC and adopted by the British Standards Institution (BSI) as a UK national standard. This batch includes ISO/IEC 14776-262:2017 (SAS Protocol Layer — 2, SPL-2) and ISO/IEC 14776-323:2017 (SCSI Block Commands — 3, SBC-3), covering the two core layers of the Serial Attached SCSI (SAS) transport and the disk block-device command set.
💡 SCSI is the “common language” of storage: hosts communicate with storage devices (disks, tapes, optical drives, RAID controllers) through a unified SCSI command set, while SAS/SPL defines the high-speed serial transport protocol layer — the cornerstone of data-centre storage interconnection.
1. Series Overview and Positioning 📌
The ISO/IEC 14776 series (corresponding to the SCSI standard architecture) comprises multiple parts specifying the SCSI architecture model (SAM), the primary command set (SPC), device-type command sets (e.g. SBC for block commands, SSC for streaming commands), and various transport protocol layers (e.g. SAS’s SPL, Fibre Channel’s FCP). The parts in this batch cover the “transport layer” and “block command set” layers.
| Standard | Title | Year | Core Content |
|---|---|---|---|
| BS ISO/IEC 14776-262:2017 | SAS Protocol Layer — 2 (SPL-2) | 2017 | SAS transport, connections, ports, links, and protocol procedures |
| BS ISO/IEC 14776-323:2017 | SCSI Block Commands — 3 (SBC-3) | 2017 | Disk block-device command set: read/write, inquiry, and status |
⚠️ SCSI terms, symbols, and conventions (e.g. UML notation) run throughout the series; both SBC-3 and SPL-2 reference the SCSI architecture model (SAM) and primary command set (SPC), so grasp the layered architecture when studying.
2. SAS Protocol Layer 2 (Part 262) 🔌
Part 262 defines the Serial Attached SCSI (SAS) protocol layer 2, specifying the protocol behaviour above the SAS physical layer and below the SCSI transport layer:
- SAS transport: frame formats, sequences, and transfer procedures of the SAS link.
- Connections and ports: PHY, link, port, and connection establishment/teardown rules.
- Addressing and routing: SAS addresses, zones, and routing rules.
- Error handling and reset: link error recovery, reset, and device-management procedures.
💡 SAS supports point-to-point high-speed serial connections and expansion (SAS expanders), while remaining interoperable with SATA devices — the mainstream interconnection scheme for enterprise disk arrays and server storage.
3. SCSI Block Commands 3 (Part 323) 💿
Part 323 defines the SCSI command set for direct-access block devices (disks):
- Core commands: READ, WRITE, READ CAPACITY, and others.
- Write protection and caching: write cache, write protection, and synchronisation commands.
- Verification: VERIFY, PREFETCH, and related commands.
- Status and parameters: mode parameters, log parameters, and status reporting.
| Command Category | Typical Commands | Purpose |
|---|---|---|
| Data transfer | READ, WRITE | Read/write user data blocks |
| Capacity and medium | READ CAPACITY, MODE SELECT | Query capacity, set medium parameters |
| Verification and cache | VERIFY, SYNCHRONIZE CACHE | Verify data, flush cache |
| Status and log | REQUEST SENSE, LOG SENSE | Error and log reporting |
4. Key Technical Requirements ⚙️
- The SAS transport layer shall follow SPL-2 frame formats, connection, and routing rules for reliable links.
- SCSI block commands shall follow SBC-3 command descriptors and parameter-page definitions for cross-device compatibility.
- Commands shall correctly handle auto-sense, error recovery, and cache consistency.
- The UML notation conventions for class diagrams, object diagrams, and constraints shall be consistent for readability and implementability.
⚠️ Cache consistency for block devices is critical: unsynchronised write caches can cause data loss after power failure — correctly use cache synchronisation and write-barrier commands.
5. Engineering Practice 🛠️
- Enterprise storage: SAS disks and RAID controller interconnection.
- Server storage interfaces: SAS host bus adapters (HBAs) and backplanes.
- Storage driver development: implement device drivers and firmware per the SBC-3 command set.
- Interoperability testing: verify cross-vendor compatibility of SPL-2 transport and SBC-3 commands.
6. Frequently Asked Questions ❓
❓ Q: How do SAS and SCSI relate?
A: SAS (Serial Attached SCSI) is a serial transport implementation of SCSI; SPL-2 defines its protocol layer, while SCSI command sets (e.g. SBC-3) run on top of the SAS transport.
A: SAS (Serial Attached SCSI) is a serial transport implementation of SCSI; SPL-2 defines its protocol layer, while SCSI command sets (e.g. SBC-3) run on top of the SAS transport.
❓ Q: What does SBC-3 define?
A: SBC-3 defines the SCSI command set for direct-access block devices (disks), including read, write, capacity inquiry, verify, and cache synchronisation commands.
A: SBC-3 defines the SCSI command set for direct-access block devices (disks), including read, write, capacity inquiry, verify, and cache synchronisation commands.
❓ Q: What is a SAS expander?
A: A SAS expander is a device for expanding SAS topology, allowing multiple PHYs to connect and route, building large storage subsystems.
A: A SAS expander is a device for expanding SAS topology, allowing multiple PHYs to connect and route, building large storage subsystems.
🔍 Q: Why does write-cache consistency matter?
A: Unsynchronised write caches can lose data after power failure. Use cache synchronisation commands (e.g. SYNCHRONIZE CACHE) and write barriers correctly to ensure data durability.
A: Unsynchronised write caches can lose data after power failure. Use cache synchronisation commands (e.g. SYNCHRONIZE CACHE) and write barriers correctly to ensure data durability.