Proof-of-capacity

Proof-of-capacity (PoC), also known as proof-of-space (PoSpace), is a means of showing that one has a legitimate interest in a service (such as sending an email) by allocating a non-trivial amount of memory or disk space to solve a challenge presented by the service provider. The concept was formulated by Dziembowski et al. in 2015 and independently by Ateniese et al. Proofs of capacity are very similar to proofs of work, except that instead of computation, storage is used. Proof-of-capacity is related to, but also considerably different from, memory-hard functions and proofs of retrievability.

Description
A PoC-based mining algorithm is based on the amount of space a miner has on their hard drive, not the amount of power or coins you have. This method is achieved by piece of data to the verifier to prove that is reserved for a certain amount of space. The impact on both CPU and hard drive is minimal. PoC is said to be fairer and greener then alternatives due to the general-purpose nature of storage and the lower energy cost.

Definitions
This method of mining was independently dubbed as farming by varies projects. It has two well established definitions based how it's used. The latter is used to efficiently store data in a blockchain-based cloud without the overheard of rising power costs.
 * 1) Renting hard drive space to earn cryptocurrency.
 * 2) Renting hard drive space to store data on a blockchain in order to earn cryptocurrency.

Burst
PoC was first publicly implemented into a cryptocurrency with Burstcoin, founded in August 2014. Burst claims to favor smaller miners by design, making transaction costs cheaper and the network more decentralized. As of 2018, profitability is now measured in the terabytes.

Chia
Chia is an upcoming cryptocurrency developed by the creator of BitTorrent. It's based on both proofs of capacity and time. Chia will be released in testing in Q2 2018 and publicly by Q4 2018.

Storj
Storj is a Amazon S3-like cloud platform that uses a blockchain to store data. It uses proof-of-capacity to both mine and store data.

SAFE
SAFE is a decentralized web platform based on a blockchain. It uses a variant of proof-of-capacity known as proof-of-resource to verify the farmer is holding the data and supplying when needed.

Advantages

 * Cheap Storage costs are cheap. 1 terabyte internal hard drive costs roughly $40.
 * Efficient Farming uses no power on both the hard drive and CPU.
 * ASIC-resistant Rewards is determined by the amount of space allocated, not processing power or coins in wallet.

Disadvantages

 * Increased space requirements Although cheap, more storage may be required if the algorithm doesn't distribute the load evenly and fairly.
 * Susceptible to grinding attacks May suffer from repeated grinding attacks if not properly protected.