NVMe Flexible Data Placement: Placement, Reclaim Units, and Events

01 Why group data for reclamation

NSID

Namespace Identifier, a controller-visible numeric handle for a namespace; the identifier is not the namespace object itself.

RGID

Reclaim Group Identifier; selects a Reclaim Group.

RGIF

Reclaim Group Identifier Format; the number of high PID bits allocated to RGID.

01.01.FDP lets the host organize writes around data usage to reduce valid-data movement during reclamation. It operates within an Endurance Group. LBA selects a logical address, while a Placement Identifier follows a namespace mapping to the Reclaim Unit currently receiving data.

Placement Identifier

Placement Identifier; A 16-bit PID containing PHNDL and, according to the configuration, RGID.

Endurance Group

The media-management scope to which an FDP configuration applies.

Reclaim Unit

Reclaim Unit; A media unit, abbreviated RU, that receives a group of data in the FDP model.

02 Isolation before and after internal relocation

Persistently Isolated

After relocation a destination RU contains only data originally written through the same RUH.

Initially Isolated

New writes are separated; relocation may combine data from different handles of this type.

02.01.An RU is referenced by at most one RUH, so new writes using different handles start separately. Initially Isolated handles allow relocated data from different handles of that type to be combined within the same Reclaim Group. Persistently Isolated handles require a destination RU containing only data written through the same handle.

Reclaim Group

Reclaim Group; A group, abbreviated RG, containing Reclaim Units.

03 Read configurations and the PID layout

03.01.LID 20h lists configurations for an Endurance Group. Use the log size and each DSZE, then inspect FDPCV, NRG, NRUH, RUHT, RUNS, and related fields. RGIF assigns the high bits of the 16-bit PID to RGID and the remaining low bits to PHNDL. A configuration index is not a byte offset.

offset

A displacement from a structure’s start; check the unit, usually bytes here.

FDPCV

FDP Configuration Valid; whether the candidate is currently available.

index

An item’s position in a list, distinct from its byte displacement.

DSZE

Descriptor Size; configuration-descriptor bytes including padding.

NRUH

Number of Reclaim Unit Handles; a direct count.

RUHT

Reclaim Unit Handle Type; selects the relocation-isolation rule.

RUNS

Reclaim Unit Nominal Size in bytes.

LID

Log Page Identifier: selects the type of log page to read.

NRG

Number of Reclaim Groups; a direct count.

04 Enable the Endurance Group configuration

FDPCIDX

FDP Configuration Index; a zero-based list index.

ENDGID

Endurance Group Identifier; identifies the target group.

FDPE

Flexible Data Placement Enable; the group enable bit in FID 1Dh.

FDPS

Flexible Data Placement Support; the controller capability bit.

SV

Save; requests saving a Set Features value.

04.01.Check FDPS and valid configurations, ensure the target Endurance Group has no namespaces, then set FID 1Dh with SV=1, FDPE and FDPCIDX. After a value change, refresh data-format information before creating namespaces. A successful feature-value change also clears that group’s FDP events and statistics.

FID

Feature Identifier: selects the Feature to read or configure.

05 Create the namespace handle mapping

05.01.NPHNDLS and the Placement Handle List define a namespace-local PHNDL→RUHID mapping. A host may list distinct RUHs; NPHNDLS=0 asks the controller to choose one for PHNDL 0. Namespaces sharing an RUH must use the same data format, and controller-selected and explicitly selected handles have allocation restrictions.

controller

Controller, the entity that implements the NVMe interface, fetches commands, and reports completions.

NPHNDLS

Number of Placement Handles explicitly supplied at namespace creation.

06 Two enablement steps and three Write cases

I/O

Input/Output, the class of data operations performed on a namespace.

06.01.Within an FDP-enabled group, enable the namespace’s Data Placement Directive before explicitly specifying a PID. Enable uses outer Identify DTYPE=00h and target DTYPE=02h in CDW12. Data Placement has no Send/Receive operations of its own. Writes without a directive use PHNDL 0 and a controller-selected Reclaim Group.

DTYPE

Directive Type; selects the management or I/O directive interpretation.

07 Follow one Write, including invalid PIDs

07.01.An explicit-placement Write uses DTYPE=02h and DSPEC=PID while LBA and block count define the logical data. If RGID or PHNDL is invalid, the controller chooses an accessible RG/RUH and records Invalid Placement Identifier if enabled on the selected RUH. This Write rule does not apply to Update.

DSPEC

Directive Specific; carries the PID for Data Placement I/O.

08 Inspect available writes and remaining time

08.01.I/O Management Receive MO=01h reports each namespace PHNDL for each RG. NVM descriptors contain PID, RUHID, EARUTR in seconds, and RUAMW in logical blocks. Each descriptor reflects processing-time state, may not reflect outstanding commands, and need not match nominal RUNS.

MO

Management Operation in I/O Management commands.

09 Request a fresh RU without erasing old data

09.01.I/O Management Send MO=01h takes a PID list and moves a written RU reference to an empty RU; an already-empty RU may be retained or changed. Invalid PIDs or excess limits cause rejection, but partial updates may already have occurred. Writes processed concurrently may use the RU before or after the update.

10 Usage reports allocation, not remaining capacity

10.01.LID 21h reports RUHA by RUHID: zero unused by namespaces, one explicitly host-selected, and two controller-selected. The Endurance Group list has at most one type-2 entry. It does not report available writes per RG; use I/O Management Receive for that.

RUHA

Reclaim Unit Handle Attributes; allocation-origin classification in Usage.

11 Measure writes over the same interval

11.01.LID 22h records host writes in HBMW, host plus internal controller writes in MBMW, and erased bytes in MBE, with the relevant data/metadata accounting. Compare deltas within the same configuration interval to observe write amplification as ΔMBMW/ΔHBMW. Do not add erased bytes to the numerator or subtract across a reset of the counters.

metadata

Additional stored information considered alongside data bytes in accounting and formats.

HBMW

Host Bytes with Metadata Written; host write accounting in bytes.

MBMW

Media Bytes with Metadata Written; includes relevant internal writes.

MBE

Media Bytes Erased; erased-byte accounting.

11.02.For NVM, HBMW/MBMW include User Data Out Commands, Write Zeroes and Write Uncorrectable; an equal-size host payload is not the criterion for accounting.

User Data Out Commands

A command category writing user data to media, not necessarily an equal-size PCIe payload.

12 Enable events on handles before reading records

12.01.FID 1Eh selects an RUH through NSID plus PHNDL and enables or disables the listed event types. Get returns supported types and enablement; Set supplies a type list. Namespaces sharing the RUH share the effect of its settings. LID 23h contains events that actually occurred.

13 Interpret event causes, locations and moved data

NSIDV

NSID Valid in an event.

LBAV

LBA Valid; validity of the example relocated LBA.

PIV

Placement Identifier Valid in an event.

LV

Location Valid; validity of event RGID and RUHID.

13.01.LID 23h selects either host or controller events and returns 64-byte records in occurrence order. Read ETYP, then PIV/NSIDV/LV to determine which identifiers are valid. The NVM Media Reallocated extension adds LBAV, NLBAM and one example LBA. Timestamp values are not guaranteed to increase.

NLBAM

Number of LBAs Moved; zero and FFFFh have special meanings.

ETYP

Event Type; the event code inside a record.

14 Separate data invalidation, RU switching and reset

AD

Attribute–Deallocate in Dataset Management.

14.01.Reducing reclamation cost requires tracking the useful lifetime of data written into an RU. When no longer needed, Dataset Management AD=1 can identify its LBA ranges. RUH Update changes the reference for future writes. After reset, refresh configuration, event settings, PID state and cache information; preserved enablement does not imply unchanged RU state.