NVMe Base 2.4: Namespace Management: capacity, creation, and attachment

01 Capacity values and allocation granularity

01.01.Establish capacity units before comparing required capacity relationships with recommended allocation granularities. The former govern validity; the latter reduce wasted allocation.

NCAP

Namespace Capacity, the maximum logical blocks that may be allocated to the namespace at any time.

NSZE

Namespace Size, the total logical-block count whose LBA range is zero through NSZE minus one.

NUSE

Namespace Utilization, the logical blocks currently allocated in the namespace.

01.02.Namespace Size (NSZE) is the total logical-block range from LBA zero through n minus one; Namespace Capacity (NCAP) is the maximum allocatable blocks at any time; and Namespace Utilization (NUSE) is the number currently allocated. NSZE is always at least NCAP, which is at least NUSE.

THINP

Thin Provisioning, the NSFEAT bit governing whether NCAP may be below NSZE and whether the controller tracks NUSE.

NCG

Namespace Capacity Granularity, the controller's preferred NCAP allocation granularity in bytes.

NSG

Namespace Size Granularity, the controller's preferred NSZE allocation granularity in bytes.

02 Data required to create a namespace

02.01.Base Figure 448 defines a 4096-byte envelope, while NVM Command Set Figure 134 defines NVM fields and the Placement Handle List within the first 768 bytes. The host selects operation and command set through SEL/CSI, then fills NSZE, NCAP, format, protection, sharing, and group identifiers. Reserved regions are zeroed, while Protection Information and FDP have separate capability gates.

Protection Information

PI: protection fields containing a Guard and tags for checking data and its associated information.

CSI

I/O Command Set Identifier: selects an I/O command set; NVM uses 00h.

FDP

Flexible Data Placement, a capability connecting data-placement hints with media-reclamation management.

NVM

Non-Volatile Memory, memory that retains data without power.

SEL

Select; the Namespace Management create/delete/restore selector, distinct from Get Features SEL.

02.02.Create uses NSID zero, SEL 0h, and CSI 00h for the NVM Command Set. DPTR identifies a 4096-byte structure: bytes 0:511 are I/O-Command-Set-specific, 512:1023 are reserved, and 1024:4095 are vendor specific. The host clears reserved bytes to zero.

DPTR

Data Pointer, the SQE field identifying a command data buffer.

I/O

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

SIOCS

Specified I/O Command Set, the Base create-buffer region at bytes 0:511 containing fields for the selected I/O Command Set.

03 Creating, attaching, and using a namespace

03.01.Create, Attach, Detach, and Delete change two dimensions: whether the namespace is allocated and whether a controller is attached. After Create returns NSID in CQE DW0, the object is allocated but no controller is attached. A Controller List in Attach establishes access. Detach preserves capacity, while Delete makes the NSID unallocated.

CQE

Completion Queue Entry, one completion-result structure in a CQ.

03.02.The complete Namespace Management capability consists of Namespace Management and Namespace Attachment. A supporting controller shall implement both, set OACS.NMS to one, and support the Attached Namespace Attribute Changed event; the Allocated event is a should, while Namespace Granularity and Restore Default are may capabilities.

OACS.NMS

The Namespace Management Supported bit in Optional Admin Command Support; one advertises the complete Manage-plus-Attach capability.

04 Deletion and restoration of default configuration

04.01.Delete All and Restore Default are different operations. Delete with NSID FFFFFFFFh succeeds even when no namespaces exist. Restore Default requires RDNCS, SEL 2h, and an empty subsystem inventory. Before successful completion, the controller applies defaults for the current active firmware image and sets DNCS to one.

RDNCS

Restore Default Namespace Configuration Supported, the capability bit advertising the Restore Default operation.

DNCS

Default Namespace Configuration Status, indicating whether the namespace configuration matches the active firmware image defaults.

04.02.Delete NSID selects a created namespace, while FFFFFFFFh means delete all and succeeds even when no namespace exists. Delete removes the namespace and has a detach side effect; the host should detach it from every controller first so events and outstanding-I/O behavior remain controlled.

05 Namespace change notifications and rediscovery

05.01.Attached and Allocated Namespace Attribute Changed notices correspond to different inventories. Create normally changes the Allocated list, Attach/Detach changes the Active list, and Delete may change both. The event code is not the new list, so the host reissues Identify with the appropriate CNS. Delete reporting also distinguishes the processing controller from other controllers.

CNS

Controller or Namespace Structure: the Identify selector for a response data structure.

05.02.Create changes the Allocated Namespace ID list, attach/detach changes the Active Namespace ID list, and delete may change both. When the corresponding notice is enabled, the host reissues Identify after the asynchronous event rather than inferring inventory from the event code alone. Section 8.1.17.2 distinguishes the controller processing delete from the other controllers.