繁體中文 00.01.When data is difficult to recover, should a controller spend more effort or use less recovery before returning a result to the host? Read Recovery Level provides this tradeoff. Start with levels and affected scope, then connect capability bits, configuration, readback and reset behavior so each field answers a concrete question.controllerThe interface entity that receives and processes host NVMe commands.scopeThe objects sharing a setting, such as one NVM Set or an entire subsystem.HostThe system running the operating system and issuing NVMe commands. The main ideas 01Understand the tradeoff01-01Levels adjust read-error recovery effort. Compare maximum, default and minimum recovery, separating...
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NVMe Read Recovery Level: Recovery Effort, Scope, and Configuration
00.01.When data is difficult to recover, should a controller spend more effort or use less recovery before returning a result to the host? Read Recovery Level provides this tradeoff. Start with levels and affected scope, then connect capability bits, configuration, readback and reset behavior so each field answers a concrete question.
- controller
- The interface entity that receives and processes host NVMe commands.
- scope
- The objects sharing a setting, such as one NVM Set or an entire subsystem.
- Host
- The system running the operating system and issuing NVMe commands.
The main ideas
Understand the tradeoff
01-01Levels adjust read-error recovery effort. Compare maximum, default and minimum recovery, separating policy from measured latency and successful recovery.
- default
- The default value; RRL defaults to level 4, independently of the current value.
Find who shares the setting
02-01Determine whether sets exist and which namespaces share a set. This identifies the affected spaces and those that cannot independently choose different levels.
Connect capability, request and result
03-01Discover supported levels, choose the target and value, then read back using the correct format. Level support, current value and saveability answer different questions.
- current
- The feature value currently in use.
Follow the setting over time
04-01Use successful completion to distinguish earlier and later commands. Then consider reset scope, current and saved values to explain when a new or restored value applies.
- saved
- The saved feature value used in applicable restoration cases.
00.02.Assumes operating systems, computer organization and basic SSD knowledge. Sets 7/9, namespaces A/B/C and encoded values are illustrative and depend on actual device capabilities.
00.03.Follow one set’s configuration, then compare easily confused values and scopes. The goal is to explain which spaces share a setting and what each query actually answers.
Connecting the main ideas
00.04.Follow one question: to use minimum recovery for reads in set 7, what must be discovered, which parameters are needed, and how can the current setting be confirmed?
00.05.The comparisons distinguish identical numbers in different result formats and identical settings under different reset scopes.
A forward route through the specification
R-1Establish the overview first, then open the Base PDF: 499 → 715–716, with one forward jump. These are PDF viewer page numbers. On shared pages, start at the named heading and stop before the next section. The Chinese tutorial explains the necessary references without repeated detours.
| Page sequence | Primary scope | Focus and stop heading |
|---|---|---|
| R1 · Base PDF 499 | §5.2.30.1.12 | Start at Read Recovery Level Config. Explain set/subsystem scope, connect Figures 484/485 with set 7 and level 15, then show Get’s return location and the SEL=3 exception. Stop before §5.2.30.1.13. |
| R2 · Base PDF 715–716 | §8.1.23 | Start at Read Recovery Level. Connect configuration to the recovery tradeoff, namespace inheritance and the LR limitation. Use Figure 755 to conclude with level direction and mandatory versus optional support. Stop before §8.1.24. |
- namespace
- A logical storage space accessed by a host through a controller.
- SEL
- Select; chooses current, default, saved or supported-capability information in Get Features.
- LR
- Limited Retry; I/O-command recovery control whose interaction with RRL is implementation-specific.
01 How much recovery should a difficult read receive?
01.01.Read Recovery Level (RRL) balances read completion time against recovery effort. When the feature is supported, level 4 is the mandatory default and level 15 is mandatory Fast Fail. Optional level 0 provides maximum recovery. Higher levels provide less recovery; interaction with an I/O command’s LR field is implementation-specific.
- Fast Fail
- The name of RRL level 15, providing minimum recovery without a fixed completion-time guarantee here.
- I/O
- Input/Output, the class of data operations performed on a namespace.
- RRL
- Read Recovery Level; selects the recovery effort applied to difficult reads.
Sources: Base 2.4 §8.1.23
Source: NVME-BASE-2.4, Rev. 2.4, §8.1.23, printed pages 689-690, PDF pages 715-716
02 Which storage spaces share one setting?
- NVM subsystem
- An NVMe storage subsystem that may contain multiple controllers and namespaces.
- NVM Set
- A logically separate collection of non-volatile memory that may contain multiple namespaces sharing an RRL.
- NVM
- Non-Volatile Memory, memory that retains data without power.
02.01.With NVM Sets, FID 12h has NVM Set scope with one RRL for the selected set. Without NVM Sets, the feature has NVM subsystem scope and all namespaces use one level. Changing RRL does not change their stored data.
- FID
- Feature Identifier: selects the Feature to read or configure.
Sources: Base 2.4 §5.2.30.1.12
Source: NVME-BASE-2.4, Rev. 2.4, §5.2.30.1.12, printed pages 473, PDF pages 499
03 Read capability bits before choosing a level
| Candidate level | Test in RRLS=8011h | Value encoded in RRL |
|---|---|---|
| 0 | Bit 0 = 1: supported | 0h |
| 4 | Bit 4 = 1: supported | 4h |
| 8 | Bit 8 = 0: unsupported | Do not select |
| 15 | Bit 15 = 1: supported | Fh |
- bitmap
- A set of bits, each representing a separate yes/no property rather than an item count.
- RRLS
- Read Recovery Levels Supported; each bit reports support for its corresponding level.
03.01.Identify Controller reports feature support in CTRATT.RRLVLS and individual levels in the 16-bit RRLS bitmap. CTRATT.NSETS selects set versus subsystem scope. ONCS.SSFS controls support for nonzero SEL and SV. These capabilities answer different questions.
- CTRATT
- Controller Attributes; a bitmap in Identify Controller.
- RRLVLS
- Read Recovery Levels; the CTRATT bit reporting RRL support.
- NSETS
- NVM Sets; the CTRATT bit reporting NVM Set support.
- ONCS
- Optional NVM Command Support; this report uses its SSFS capability bit.
- SSFS
- Save and Select Feature Support; indicates support for nonzero SV and SEL.
- SV
- Save; the Set Features bit requesting a saved setting.
Sources: Base 2.4 §5.2.14.2.1
Source: NVME-BASE-2.4, Rev. 2.4, §5.2.14.2.1, printed pages 344-347,371-372, PDF pages 370-373,397-398
04 Encode the target set and requested level
04.01.Set Features selects Read Recovery Level Config with FID 12h. CDW11 bits 15:0 carry NVMSETID; CDW12 bits 3:0 carry RRL. No data buffer is used, and NSID should be zero for the NVM Set or subsystem scope.
- NVMSETID
- NVM Set Identifier; a 16-bit set identifier distinct from a namespace NSID.
- buffer
- A data buffer; this feature does not require one for its attributes.
- NSID
- Namespace Identifier, a controller-visible numeric handle for a namespace; the identifier is not the namespace object itself.
- CDW
- CDW (Command Dword): a 32-bit command field. In CDW10, 10 is the field index, not a byte offset.
Sources: Base 2.4 §5.2.30.1.12
Source: NVME-BASE-2.4, Rev. 2.4, §5.2.30.1.12, printed pages 473, PDF pages 499
05 Interpret returned DW0 according to SEL
- NSSPEC
- NS Specific; one indicates namespace scope, while zero does not identify another scope.
- CHANG
- Changeable; reports whether feature values can be changed.
- SVBL
- Saveable; reports whether the feature values can be saved.
- CQE
- Completion Queue Entry; reports a command result.
05.01.Successful Get Features with SEL other than 3 returns RRL in CQE DW0 using Figure 485. SEL=3 instead returns Supported Capabilities: CHANG, NSSPEC and SVBL. The same DW0 value can have entirely different meanings; the level bitmap remains in Identify Controller.
Sources: Base 2.4 §5.2.30.1.12
Source: NVME-BASE-2.4, Rev. 2.4, §5.2.30.1.12, printed pages 473, PDF pages 499
06 Use successful completion as the settings boundary
06.01.Commands already submitted may or may not use a newly changed feature value. Commands submitted after successful Set Features completion must use the new settings. To separate two read batches clearly, the host should let outstanding commands complete before changing the feature.
Sources: Base 2.4 §5.2.30.1
Source: NVME-BASE-2.4, Rev. 2.4, §5.2.30.1, printed pages 459, PDF pages 485
07 Persistence depends on saveability and reset scope
| Capability and prior values | Applicable event condition | Resulting current |
|---|---|---|
| Non-saveable; current=15 | Power cycle or reset: FID12h is persistent | 15 |
| Saveable; current=15, saved=4 | Whole-subsystem reset under Figure126 | 4: from saved |
| Saveable; current=15, no saved value | Whole-subsystem reset under Figure126 | 4: from default |
| Saveable; current=15, saved=4 | Subset reset under Figure127; set/subsystem scope | 15: unchanged |
07.01.Figure 466 lists persistence as Yes for FID 12h, but that column applies only to non-saveable features. If the feature is saveable, distinguish current from saved and use the reset scope: Figure 126 for whole-subsystem scope, Figure 127 for a subset. Resetting one controller in a multi-controller subsystem retains the shared current value. An applicable whole-subsystem reset instead restores saved, or default if no saved value exists.
Sources: Base 2.4 §4.4
Source: NVME-BASE-2.4, Rev. 2.4, §4.4, printed pages 167-169, PDF pages 193-195
Where to continue in the specification
08.01.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 |
|---|---|
| How much recovery should a difficult read receive? | Base 2.4 §8.1.23 |
| Which storage spaces share one setting? | Base 2.4 §5.2.30.1.12 · Base 2.4 §8.1.23 · Base 2.4 §3.2.2 |
| Read capability bits before choosing a level | Base 2.4 §5.2.14.2.1 |
| Encode the target set and requested level | Base 2.4 §5.2.30.1.12 · Base 2.4 §4.4 · Base 2.4 §5.2.30 |
| Interpret returned DW0 according to SEL | Base 2.4 §5.2.30.1.12 · Base 2.4 §5.2.12 |
| Use successful completion as the settings boundary | Base 2.4 §5.2.30.1 · Base 2.4 §4.4 |
| Persistence depends on saveability and reset scope | Base 2.4 §4.4 · Base 2.4 §5.2.30 |
Check your understanding
1. With RRLS=8011h, is level 8 available, and should level 15 be encoded as 8000h?
09.01.No: bit 8 is zero. For level 15, check RRLS bit 15, then encode Fh in the four-bit RRL field. The value 8000h is a support-bit mask, not the level code.
Sources
Source: NVME-BASE-2.4, Rev. 2.4, §5.2.14.2.1, printed pages 344-347,371-372, PDF pages 370-373,397-398
Source: NVME-BASE-2.4, Rev. 2.4, §5.2.30.1.12, printed pages 473, PDF pages 499
2. Can A’s NSID select level 15 only for A when A and B share one set?
09.02.No. RRL is shared at set scope and NVMSETID selects the set. NSID should be zero for this scope; nonzero does not narrow the effect and causes the command to be aborted.
Sources
Source: NVME-BASE-2.4, Rev. 2.4, §5.2.30.1.12, printed pages 473, PDF pages 499
Source: NVME-BASE-2.4, Rev. 2.4, §4.4, printed pages 169, PDF pages 195
3. Why is successful Get DW0=4 not enough to conclude that the current level is 4?
09.03.Check SEL: zero returns current, while three makes 4 the CHANG=1/NSSPEC=0/SVBL=0 capability combination. With SEL=1, it is the default value.
Sources
Source: NVME-BASE-2.4, Rev. 2.4, §5.2.30.1.12, printed pages 473, PDF pages 499
Source: NVME-BASE-2.4, Rev. 2.4, §5.2.12, printed pages 209-212, PDF pages 235-238
4. After Set level 15 succeeds, must both an outstanding read A and a subsequently submitted read B use 15?
09.04.B must use the new setting; already-submitted A may or may not use it. Level 15 also has no fixed millisecond bound here. Complete A before changing settings when a clear batch boundary is needed.
Sources
Source: NVME-BASE-2.4, Rev. 2.4, §5.2.30.1, printed pages 459, PDF pages 485
Source: NVME-BASE-2.4, Rev. 2.4, §8.1.23, printed pages 689-690, PDF pages 715-716
5. Does successfully setting 15 with SV=0 mean every reset restores 4?
09.05.Not necessarily. Non-saveable FID 12h is persistent. A saveable instance depends on reset scope: Figure 126 restores saved or default, while Figure 127 retains current for set/subsystem scope.
Sources
Source: NVME-BASE-2.4, Rev. 2.4, §4.4, printed pages 167-169, PDF pages 193-195
Source: NVME-BASE-2.4, Rev. 2.4, §5.2.30, printed pages 458-459, PDF pages 484-485
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