繁體中文 00.01.This report explains how a host tells an SSD which data belongs together. Establish the relationships among host, controller, namespace, and stream, then follow capability checks, enablement, allocation, writes, and release. Finally examine identifier sharing across paths and the meaning of SWS/SGS for data organization.controllerController, the entity that implements the NVMe interface, fetches commands, and reports completions.namespaceNamespace, a formatted quantity of non-volatile memory accessed by a host through a controller.HostThe system running the operating system and issuing NVMe commands.SGSStream Granularity Size; allocation granularity measured in SWS units.SWSStream Write Size; the optimal write alignment/size unit, measured in logical blocks for...
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NVMe Directives and Streams: Host Grouping, Writes, and Resources
00.01.This report explains how a host tells an SSD which data belongs together. Establish the relationships among host, controller, namespace, and stream, then follow capability checks, enablement, allocation, writes, and release. Finally examine identifier sharing across paths and the meaning of SWS/SGS for data organization.
- controller
- Controller, the entity that implements the NVMe interface, fetches commands, and reports completions.
- namespace
- Namespace, a formatted quantity of non-volatile memory accessed by a host through a controller.
- Host
- The system running the operating system and issuing NVMe commands.
- SGS
- Stream Granularity Size; allocation granularity measured in SWS units.
- SWS
- Stream Write Size; the optimal write alignment/size unit, measured in logical blocks for NVM.
The main ideas
Why group data?
01-01A Stream Identifier adds a relationship between writes while LBA still selects the destination. A stream is not another namespace.
- Stream Identifier
- A host-selected 16-bit grouping label interpreted with namespace, host identity, and sharing configuration.
Management and data flow
02-01Capability queries, identity registration, enablement, and allocation prepare the system; tagged Writes actually use streams. Management commands are not repeated for every payload.
- payload
- Data carried by a command transfer, distinct from command fields and completion values.
Identity and resource sharing
03-01Host Identifier and SSID determine which paths share a stream. NSA, NSSA, and NSO distinguish capacity, the resource pool, and current use.
- Host Identifier
- The identity used to associate controllers with a host, distinct from a Stream Identifier.
- NSSA
- NVM Subsystem Streams Available; the non-exclusively allocated pool size, not its idle-resource count.
- SSID
- Shared Stream Identifiers; controls sharing of equal identifiers across nonzero host identities within a namespace.
- NSA
- Namespace Streams Allocated; the exclusive allocation for the namespace and visible host association, also returned by Allocate.
- NSO
- Namespace Streams Open; the open count for the selected namespace and visible host association.
Size and lifetime
04-01SWS/SGS guide writes and deallocation. Release, disablement, and reset affect tracking state independently of the continued existence of user data.
00.02.The assumed background is operating systems, computer organization, and basic SSD concepts. Examples use PCIe controllers and cover the specified Directives/Streams scope, excluding Data Placement §8.1.9.4. Counts, LBAs, and host labels are illustrative; actual capabilities come from the device.
- Directives
- A mechanism for exchanging additional host/controller information, with type-specific management and I/O semantics.
- Streams
- A Directive that groups related writes using stream identifiers without selecting physical NAND addresses.
- PCIe
- PCI Express, the transport and device interconnect used by an NVMe memory-based controller.
Connecting the main ideas
00.03.Follow a complete data-use cycle: decide which data is related, verify support, establish host identities and enablement, and choose a resource source. Writes create stream activity; release ends an identifier’s current grouping when it is no longer needed.
00.04.Equal numbers need not mean equal things: the same Stream Identifier may belong to different hosts, and SWS=8 versus NSA=8 means data size versus resource count. The examples hold other conditions fixed while changing one factor.
The live specification route
R-1Use the examples to establish the overall picture, then open Base at PDF page 227. Proceed forward through the viewer pages below and switch once to NVM. On shared pages, stop at the named heading; use viewer pages rather than stale contents-page numbers.
- NVM
- Non-Volatile Memory, memory that retains data without power.
| Page sequence | Sections | What to explain here |
|---|---|---|
| R1 · Base PDF 227–228 | §5.2.7 → §5.2.8 | Compare DPTR, NUMD, and DTYPE/DOPER for Receive/Send; stop before §5.2.9 Firmware Commit. |
| R2 · Base PDF 534–536 | §5.2.30.1.35 → .35.1 | Read EXHID, HOSTID, zero/nonzero identity, and non-saveability under PCIe; stop before .35.2. |
| R3 · Base PDF 642–646 | §8.1.9 → §8.1.9.2 | Start at Directives. Use 702/703 for types, then 704–707 for queries and enablement. |
| R4 · Base PDF 646–653 | §8.1.9.3 | 708 sizes → 709 operations → 710 sharing → 711–715 parameters/status/allocation → the two releases. Stop before §8.1.9.4. |
| R5 · NVM PDF 175 | §5.13 | Confirm SWS is in logical blocks and finish the 4 KiB, SWS=8, SGS=4 calculation. |
- HOSTID
- The actual identity value in the Host Identifier data structure.
- DOPER
- Directive Operation; selects an operation within a directive type and command direction.
- DTYPE
- Directive Type; selects a directive. Enable uses separate outer-operation and inner-target type fields.
- EXHID
- Enable Extended Host Identifier; zero selects 64 bits and one selects 128 bits.
- DPTR
- Data Pointer, the SQE field identifying a command data buffer.
- NUMD
- Number of Dwords; for buffered transfers, encodes the count of four-byte units minus one.
01 What Streams communicates
- Check Directives/Streams support and register an appropriate nonzero Host Identifier.
- Enable Streams through the Identify Directive and inspect state and parameters.
- Use exclusive or subsystem resources; request and inspect a grant if exclusive capacity is needed.
- Issue Writes with Stream Identifiers; first use of an unopened identifier opens a stream.
- Observe open identifiers with Get Status; release an identifier to end its grouping or release resources to return the exclusive allocation.
01.01.Directives exchange additional information between the host and controller. Streams uses a host-selected Stream Identifier to associate writes with a group. The controller may use that grouping for placement or performance improvements; the identifier does not select a physical NAND address.
Sources: Base 2.4 §8.1.9, 8.1.9.1, 8.1.9.3
Source: NVME-BASE-2.4, Rev. 2.4, §8.1.9, 8.1.9.1, 8.1.9.3, printed pages 616-623, PDF pages 642-649
02 Identify the host across controller paths
02.01.FID 81h registers a Host Identifier so controllers in an NVM subsystem can recognize a common host. Equal nonzero values establish that identity; 0h does not associate a controller with other controllers. The PCIe default is 0h and this Feature is not saveable.
- NVM subsystem
- NVM subsystem, the NVMe system boundary containing controllers, ports, namespaces, and non-volatile storage resources.
- FID
- Feature Identifier: selects the Feature to read or configure.
Sources: Base 2.4 §5.2.30.1.35, 5.2.30.1.35.1
Source: NVME-BASE-2.4, Rev. 2.4, §5.2.30.1.35, 5.2.30.1.35.1, printed pages 508-510, PDF pages 534-536
03 Check support and enable Streams
- ENDIR
- Enable Directive; one enables and zero disables the type selected in CDW12.
- CDW
- CDW (Command Dword): a 32-bit command field. In CDW10, 10 is the field index, not a byte offset.
03.01.First check OACS.DIRS, then query Streams support and enablement through the Identify Directive. For Enable Directive, CDW11 selects the Identify operation and CDW12 selects Streams as the target. Namespace and Host Identifier determine the shared enable state.
- DIRS
- Directives Supported; the general capability bit in OACS.
Sources: Base 2.4 §8.1.9, 8.1.9.2, 8.1.9.3
Source: NVME-BASE-2.4, Rev. 2.4, §8.1.9, 8.1.9.2, 8.1.9.3, printed pages 616-624, PDF pages 642-650
04 Read the Directive command envelope
04.01.Directive Receive transfers data toward the host and Directive Send toward the controller, when the selected operation transfers data. CDW11 selects DTYPE/DOPER and type-specific DSPEC. For buffer transfers, NUMD encodes the Dword count minus one.
- buffer
- A memory buffer prepared by the host to supply or receive data.
- DSPEC
- Directive Specific; interpreted by type/operation and used as the Stream Identifier in Streams I/O.
- Dword
- Dword (double word): 32 bits, or 4 bytes. A word is 16 bits; for example, a zero-based dword count of 3 represents 4 Dwords, or 16 bytes.
Sources: Base 2.4 §5.2.7, 5.2.8
Source: NVME-BASE-2.4, Rev. 2.4, §5.2.7, 5.2.8, printed pages 201-202, PDF pages 227-228
05 Separate allocated capacity from open streams
- NSSO
- NVM Subsystem Streams Open; open streams using the nonexclusive pool.
- MSL
- Max Streams Limit; the subsystem-wide concurrent-open limit.
05.01.Streams uses either namespace-exclusive resources or subsystem resources not exclusively allocated. Allocate Resources requests concurrent tracking capacity; returned NSA may be smaller than NSR. A stream is opened when a later Write uses an identifier that is not already open.
- NSR
- Namespace Streams Requested; the directly encoded exclusive resource request count.
Sources: Base 2.4 §8.1.9.3, 8.1.9.3.1.1, 8.1.9.3.1.3
Source: NVME-BASE-2.4, Rev. 2.4, §8.1.9.3, 8.1.9.3.1.1, 8.1.9.3.1.3, printed pages 621-626, PDF pages 647-652
06 When equal Stream Identifiers mean the same stream
| Controller path | Nonzero Host Identifier | Stream group: SSID=0 | Stream group: SSID=1 |
|---|---|---|---|
| C1 | A | A / 7 | 7 |
| C2 | A | A / 7 | 7 |
| C3 | B | B / 7 | 7 |
| C4 | C | C / 7 | 7 |
06.01.Interpret a Stream Identifier together with namespace, Host Identifier, and NSSC.SSID. With SSID=0, equal identifiers belonging to different nonzero host identities are separate streams. With SSID=1, those hosts may share the same numbered stream in the same namespace.
- NSSC
- NVM Subsystem Stream Capability; contains identity requirements and sharing capabilities.
Sources: Base 2.4 §8.1.9.3, 8.1.9.3.1.1, 8.1.9.3.1.2
Source: NVME-BASE-2.4, Rev. 2.4, §8.1.9.3, 8.1.9.3.1.1, 8.1.9.3.1.2, printed pages 621-626, PDF pages 647-652
07 Attach a Write to a stream
07.01.A Streams Write uses DTYPE=1 and a nonzero Stream Identifier in DSPEC. First use of an unopened identifier opens a stream. DTYPE=0 or a zero Streams DSPEC omits stream grouping; handling an unenabled type also depends on whether any I/O Directive is enabled.
- I/O
- Input/Output, the class of data operations performed on a namespace.
Sources: Base 2.4 §8.1.9.1, 8.1.9.3
Source: NVME-BASE-2.4, Rev. 2.4, §8.1.9.1, 8.1.9.3, printed pages 617-623, PDF pages 643-649
08 Convert SWS and SGS into data sizes
- logical block
- An addressable unit in a namespace; its data size is determined by the active format.
- granularity
- Granularity; a size unit used for writes or allocation, not a resource count.
08.01.For the NVM Command Set, SWS is measured in logical blocks; SGS is measured in SWS units. Convert through the namespace logical block size before comparing write alignment, transfer length, and group-sized deallocation ranges.
- alignment
- Alignment; the start position is a multiple of the required unit, checked separately from transfer length.
Sources: NVM Command Set 1.3 §5.13
Source: NVME-NVM-CS-1.3, Rev. 1.3, §5.13, printed pages 175, PDF pages 175
09 Observe streams and end their use
| Event or operation | Identifier/open state | Exclusive allocation | User data |
|---|---|---|---|
| Release Identifier | Ends this use of one identifier | Retained for namespace reuse | Does not delete data |
| Release Resources | NSA=0 alone is not a complete identifier-state report | Returned; NSA becomes zero | Does not delete data |
| Disable Streams | Releases affected host identifiers | Releases affected resources | Not secure erasure |
| Format NVM | Releases affected open identifiers | Do not infer release of all capacity from this rule | Data effects follow Format |
| Delete / become write-protected | Releases namespace stream identifiers | Releases all stream resources | Tracking release is not media erasure |
09.01.Get Status reports open identifiers. Release Identifier ends the current association of one identifier while exclusive capacity can remain allocated; Release Resources returns the namespace-exclusive allocation. Disablement, Format, deletion, write protection, and reset have distinct effects.
Sources: Base 2.4 §8.1.9.2.2.1, 8.1.9.3, 8.1.9.3.1.2, 8.1.9.3.2
Source: NVME-BASE-2.4, Rev. 2.4, §8.1.9.2.2.1, 8.1.9.3, 8.1.9.3.1.2, 8.1.9.3.2, printed pages 619-627, PDF pages 645-653
Where to continue in the specification
10.01.Use the flow above to frame the problem, then open the corresponding sections for fields and full conditions. The Chinese tutorial also explains every in-scope figure with its takeaway, example, and details.
| Concept to explain | Specification sections |
|---|---|
| What Streams communicates | Base 2.4 §8.1.9, 8.1.9.1, 8.1.9.3 |
| Identify the host across controller paths | Base 2.4 §5.2.30.1.35, 5.2.30.1.35.1 |
| Check support and enable Streams | Base 2.4 §8.1.9, 8.1.9.2, 8.1.9.3 |
| Read the Directive command envelope | Base 2.4 §5.2.7, 5.2.8 |
| Separate allocated capacity from open streams | Base 2.4 §8.1.9.3, 8.1.9.3.1.1, 8.1.9.3.1.3 |
| When equal Stream Identifiers mean the same stream | Base 2.4 §8.1.9.3, 8.1.9.3.1.1, 8.1.9.3.1.2 |
| Attach a Write to a stream | Base 2.4 §8.1.9.1, 8.1.9.3 |
| Convert SWS and SGS into data sizes | NVM 1.3 §5.13 |
| Observe streams and end their use | Base 2.4 §8.1.9.2.2.1, 8.1.9.3, 8.1.9.3.1.2, 8.1.9.3.2 |
Check your understanding
1. Are SDIRS=1, NSA=3, and NSO=0 contradictory?
11.01.No. They describe support, exclusive capacity, and the current open count. The host can have enabled and allocated capacity without yet issuing a stream-tagged write.
Sources
Source: NVME-BASE-2.4, Rev. 2.4, §8.1.9, 8.1.9.2, 8.1.9.3, printed pages 616-624, PDF pages 642-650
Source: NVME-BASE-2.4, Rev. 2.4, §8.1.9.3, 8.1.9.3.1.1, 8.1.9.3.1.3, printed pages 621-626, PDF pages 647-652
2. Do C1 and C2 with Host Identifier=0h share host identity for stream 7?
11.02.No. Zero explicitly does not establish a cross-controller host association. Matching nonzero identities are needed for that common-host relationship.
Sources
Source: NVME-BASE-2.4, Rev. 2.4, §5.2.30.1.35, 5.2.30.1.35.1, printed pages 508-510, PDF pages 534-536
Source: NVME-BASE-2.4, Rev. 2.4, §8.1.9.3, 8.1.9.3.1.1, 8.1.9.3.1.2, printed pages 621-626, PDF pages 647-652
3. After receiving three exclusive resources, can another NSR=2 request make five?
11.03.No. A request while that exclusive allocation exists is rejected. Release the allocation, request the full new count, and inspect the grant. Releasing one identifier is not releasing the whole allocation.
Sources
Source: NVME-BASE-2.4, Rev. 2.4, §8.1.9.3, 8.1.9.3.1.1, 8.1.9.3.1.3, printed pages 621-626, PDF pages 647-652
Source: NVME-BASE-2.4, Rev. 2.4, §8.1.9.2.2.1, 8.1.9.3, 8.1.9.3.1.2, 8.1.9.3.2, printed pages 619-627, PDF pages 645-653
4. With 4 KiB/block, SWS=8, and SGS=4, which size recommendations are met by a 16-block Write and deallocation starting at LBA 136?
11.04.The Write start and length are multiples of eight and satisfy SWS. Stream granularity is 32 blocks; neither 136 nor 16 is a multiple of 32, so the deallocation optimization recommendation is not met. This does not declare the command invalid.
Sources
Source: NVME-NVM-CS-1.3, Rev. 1.3, §5.13, printed pages 175, PDF pages 175
5. If subsystem Get Status lists identifier 7 once, does that prove only one namespace uses it?
11.05.No. The subsystem query covers nonexclusive resources and deduplicates equal identifiers across namespaces. Query the particular NSID under the appropriate host association for its details.
Sources
Source: NVME-BASE-2.4, Rev. 2.4, §8.1.9.2.2.1, 8.1.9.3, 8.1.9.3.1.2, 8.1.9.3.2, printed pages 619-627, PDF pages 645-653
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