Glossary

Storage Hardware

• Block device: A storage object that exposes fixed-size logical blocks addressed by LBA.
• LBA: Logical Block Address. The number used to address a logical block.
• Sector: Often used casually for a logical block, commonly 512 bytes or 4096 bytes.
• NAND page: The small unit NAND can program/read.
• Erase block: The larger NAND unit that must be erased before pages can be rewritten.
• FTL: Flash Translation Layer. SSD firmware that maps host LBAs to flash locations.
• Write amplification: Extra internal writes performed by the SSD beyond host writes.
• TRIM/UNMAP: Host hint that logical blocks no longer contain useful data.
• Power-loss protection: Hardware/firmware support for completing or preserving writes across power failure.

NVMe

• Controller: NVMe device-side object that owns capabilities, queues, and namespaces.
• Namespace: NVMe logical storage range exposed by a controller.
• SQ: Submission Queue. Host memory ring where commands are placed.
• CQ: Completion Queue. Host memory ring where completions are written.
• Doorbell: MMIO register write telling the controller that a queue tail/head moved.
• Phase bit: Completion queue wrap indicator used to distinguish old entries from new ones.
• Admin queue: Queue pair used for controller management commands.
• IO queue: Queue pair used for read/write/flush/dataset-management commands.
• qpair: SPDK/NVMe shorthand for one submission queue and one completion queue.
• ANA: Asymmetric Namespace Access, used in NVMe multipath accessibility reporting.

PCIe, DMA, VFIO

• BDF: Bus:Device.Function PCI address.
• BAR: Base Address Register, often mapping device MMIO registers.
• MMIO: Memory-mapped IO, where device registers are accessed as memory addresses.
• DMA: Direct Memory Access, where a device reads/writes host memory.
• IOMMU: Hardware that translates and protects device DMA.
• IOVA: IO virtual address used by a device/DMA mapping.
• VFIO: Linux framework for safely assigning devices to userspace drivers.
• Hugepage: Large memory page used to reduce translation overhead and support pinned DMA memory.

SPDK Runtime

• Reactor: SPDK event loop usually pinned to a CPU core.
• spdk_thread: Lightweight SPDK execution context, not the same thing as a POSIX thread.
• Message: Function sent to an SPDK thread for execution on that thread.
• Poller: Callback repeatedly or periodically run by an SPDK thread.
• io_device: Object that can provide per-thread channel state.
• io_channel: Thread-local resource bundle associated with an io_device.
• NOMEM: SPDK pattern where an IO cannot allocate required resources and must be retried later.
• No blocking rule: Reactor/poller code should not sleep or run slow blocking operations.

bdev

• bdev: SPDK block device abstraction.
• Descriptor: Open handle to a bdev, including access mode and event callback.
• bdev channel: Per-thread IO channel for submitting to a bdev.
• spdk_bdev_io: Object representing one bdev IO operation.
• Module: Implementation provider for one or more bdevs.
• Virtual bdev: bdev that transforms or stacks on top of other bdevs.
• Claim: Ownership relationship used by a virtual module to protect a base bdev.
• Examine: Process where modules inspect bdevs and discover metadata-backed virtual devices.
• QoS: Rate limiting and scheduling controls around bdev IO.

Blobstore, lvol, RAID

• Blobstore: SPDK allocator/object layer over a bdev.
• Blob: Logical object managed by blobstore.
• Cluster: Blobstore allocation unit.
• Thin provisioning: Allocating physical storage only when data is written.
• Snapshot: Read-only point-in-time parent for clone relationships.
• Clone: Writable blob/lvol sharing data with a snapshot until overwritten.
• External snapshot: Snapshot data provided by an external backing device.
• lvstore: Logical volume store backed by blobstore.
• lvol: Logical volume exposed as a blobstore-backed object.
• RAID bdev: Virtual bdev combining multiple base bdevs.
• Degraded: State where redundancy or required bases are missing but some operations may continue.

Transports And Exports

• NVMe-oF: NVMe over Fabrics, carrying NVMe commands over transports such as RDMA or TCP.
• NQN: NVMe Qualified Name.
• Subsystem: NVMe/NVMe-oF object containing namespaces and host access policy.
• Listener: Network address where an NVMe-oF subsystem accepts connections.
• Poll group: Group of transport qpairs polled together.
• vhost: Userspace backend for virtio queues, often used by QEMU.
• vfio-user: Protocol for userspace device emulation over a socket, including NVMe-style exposure.

Control And Debugging

• JSON-RPC: SPDK control-plane protocol for creating, deleting, querying, and configuring objects.
• Config replay: Recreating runtime objects by replaying saved RPC-like configuration.
• --wait-for-rpc: Startup mode where SPDK waits for RPC before continuing initialization.
• Tracepoint: Low-overhead instrumentation point for debugging timing and state.
• bdev stats: Counters and latency data exposed for bdevs.
• Idempotency: Property where repeating a restore operation converges without harmful duplicates.

diskengine

• Storage-node mode: diskengine mode that owns local NVMe/lvol storage and exports volumes.
• Baremetal mode: diskengine mode that attaches remote storage and exposes it to local workloads.
• Desired state: What diskengine's database says should exist.
• Observed state: What SPDK and the host currently report.
• Reconciliation: Loop that compares desired and observed state and safely converges them.