Piece CID

A Piece CID (Content Identifier) is a unique cryptographic hash that represents a specific, fixed-size chunk of data, known as a piece, prepared for storage on the Filecoin network. It is generated by applying a SHA-256 hash to the raw bytes of the piece after it has undergone piece padding to meet the network's power-of-two size requirements (e.g., 256 KiB, 512 KiB, 1 MiB, etc.). Unlike a typical file CID, which identifies the logical structure of a file, a Piece CID identifies the exact physical data payload that a storage provider commits to storing.

The creation of a Piece CID is a prerequisite for a storage deal. Before data can be stored, a client's files are segmented into one or more pieces, each with its own Piece CID. This identifier is then included in the deal proposal and is cryptographically linked to the provider's storage commitment via Proof-of-Replication and Proof-of-Spacetime. The Piece CID serves as the fundamental unit of storage and verification, enabling the network to audit that a provider is storing the exact, agreed-upon data.

A key distinction is between a Piece CID and a Payload CID (or Data CID). The Payload CID, often a CIDv1, identifies the user's original data (e.g., a file or directory) using a codec like dag-pb. The Piece CID is derived from the padded binary representation of that data. While they represent the same underlying content, they are different hashes with different purposes: the Payload CID is for retrieval and data graphs, while the Piece CID is for storage proofs and sector sealing.

In practice, when data is retrieved from Filecoin, the storage provider returns the original payload data. The client can verify its integrity by recomputing the Payload CID. The Piece CID remains a critical backend component, ensuring the data stored in the provider's sealed sector is immutable and verifiable against the original deal terms. This two-tiered identification system is central to Filecoin's trustless storage and verifiable capacity market.

The creation of a Piece CID begins with the client's raw data. This data is prepared by a Storage Provider into a CAR (Content Addressed aRchive) file, which contains the data and its corresponding IPLD DAG structure. The CAR file is then padded to a standard size that aligns with Filecoin's sector sizes (e.g., 32 GiB or 64 GiB). This padding ensures the data fits precisely into the cryptographic compartments used for sealing and proof generation, making the Piece CID a commitment to a specific, fixed-size data segment.

Once padded, the data undergoes a SHA-256 hash to generate the Piece CID. This hash is encoded with the multicodec fil-piece (0x1012) and the multihash sha2-256 (0x12), resulting in a self-describing content identifier like baga6ea4seaq.... The fil-piece codec is critical—it signals that this CID refers to the raw, unsealed data piece, distinct from the dag-cbor or raw codecs used for the data's logical content. This process is entirely deterministic; the same data, prepared identically, will always produce the same Piece CID.

The final Piece CID and its size are the core components of a storage deal proposal on the Filecoin network. The Storage Provider commits to storing the data referenced by this specific CID. Later, during the sealing process, this Piece CID is used to generate the CommP (Commitment to the Piece), which is then embedded into the sector's Sealed CID (CommR). This chain of cryptographic commitments links the provable storage in a sector directly back to the original client data, enabling verifiable storage proofs without the provider needing to repeatedly access the raw data.

The first component, CID (Content Identifier), is a self-describing content-addressed identifier. It originates from the InterPlanetary File System (IPFS) and Filecoin ecosystems, where it serves as a unique fingerprint for any piece of data, generated using cryptographic hash functions like SHA-256. The concept of content addressing itself has roots in peer-to-peer networks and distributed systems, where identifying data by its content, rather than its location, is paramount for verifiability and persistence.

The second component, piece, is a specific construct from the Filecoin protocol. A piece is a CAR (Content Addressed aRchive) file containing user data that is prepared for storage deals with miners. The term reflects the protocol's design where data is packaged into standardized, sector-sized units for efficient storage and proof generation. The etymology ties directly to the physical metaphor of breaking data into manageable "pieces" for storage in a decentralized network.

Combined, Piece CID specifically refers to the CID of the root node of the IPLD DAG contained within a CAR file that constitutes a storage deal's payload. This precise definition distinguishes it from a regular data CID; a Piece CID identifies the entire packaged payload for a storage deal, which includes the target data plus any padding required to fill a storage sector. Its origin is inextricably linked to Filecoin's economic and cryptographic mechanisms for proving storage.

The term's adoption and standardization were driven by the need for a unambiguous identifier in storage deals and retrieval markets. It serves as the primary committed data reference in the Filecoin blockchain's Deal state and in Proof-of-Spacetime submissions. Understanding its dual etymology is key for developers interacting with Filecoin's storage abstraction layer, as it sits at the intersection of content addressing (CID) and storage logistics (piece).