Verifying Storage on Filecoin | Preparing and Transferring Data

Before a system file (such as puppy.gif) can be stored on the Filecoin network, it must first be converted into a Filecoin Piece.

In the first phase of this conversion, the system files are chunked together with UnixFS to create an IPLD DAG (directed acyclic graph). You can learn more about DAGs (a form of merkle tree) in our decentralized data structures tutorial. The payload CID of this IPLD DAG is the same as the IPFS CID and represents the root of the DAG.

The IPLD DAG is then serialized into a CAR file and bit-padded to make a Filecoin Piece. (Bit-padding adds extra bits to make the piece fit the standard size.) The piece has a unique piece CID, also known as a CommP (piece commitment).

Since the payload CID and fragment CID are cryptographic hashes of the data itself, they are unique and having the same CID guarantees that the content is identical. The same IPLD DAG will produce the same payload CID, and the same fragment will produce the same fragment CID, no matter who stores or retrieves them.

When clients negotiate a storage deal with a miner, they hire them to store a block of data, which could be a whole or partial file. Miners store the pieces from one or more clients in sectors, the basic unit of storage used by Filecoin. Sectors vary in size, and clients can store up to a certain size of data per deal. (Learn more about sector sizes and storing large files.)

The CID piece is packaged with other transaction parameters to create a transaction proposal. The transaction CID contains information about the data itself, in the form of a block CID, the identity of the miner and client, and other important transaction details.

The client sends this transaction proposal to the miner who agrees to store their data. Once the miner confirms, the client will transfer their data to the miner. Once the miner has the data and verifies that it matches the CID noted in the transaction proposal, they publish the transaction proposal on Filecoin's blockchain and commit both parties to the transaction.

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