Ethereum Classic Activates Thanos Hard Fork to Preserve GPU Mining Viability

Ethereum Classic (ETC) underwent a significant network upgrade on November 28, 2020, as the Thanos hard fork officially went live at block number 11,700,000. The upgrade, formalized as ECIP-1099, addressed a growing concern among ETC miners: the ever-expanding DAG file size that threatened to make consumer-grade GPU hardware obsolete for mining the network.

TL;DR

• Ethereum Classic activated the Thanos hard fork at block 11,700,000 on November 28, 2020
• The upgrade (ECIP-1099) reduces DAG file size by doubling the epoch length from 30,000 to 60,000 blocks
• A new mining algorithm called ETChash replaces Ethash for the ETC network
• The fork keeps 4GB GPUs viable for mining ETC for approximately three more years
• OpenEthereum, OpenETC, Classic Geth, and MultiGeth discontinued ETC support; Hyperledger Besu continues

What Is the DAG Problem?

To understand why Thanos matters, it helps to understand how Ethereum-based networks handle mining. Both Ethereum and Ethereum Classic use a mining algorithm called Ethash, which relies on a data structure known as a Directed Acyclic Graph, or DAG. The DAG file must be loaded into a GPU’s video memory (VRAM) for mining to work properly.

Here is the catch: the DAG file grows over time. For every 30,000 blocks — roughly every 100 hours — a new epoch begins and the DAG increases in size. By November 2020, the DAG had reached approximately 3.91 GB at epoch 372. This meant that GPUs with 4 GB of VRAM were rapidly approaching the point where they could no longer mine Ethereum Classic, as the DAG file would soon exceed their memory capacity.

Once the DAG file surpasses a GPU’s VRAM, that hardware becomes effectively useless for mining on that network. This creates a hardware arms race that favors miners with expensive, high-memory GPUs, undermining the decentralization that proof-of-work mining is supposed to provide.

How Thanos Solves the Problem

The Thanos upgrade implements a straightforward but effective solution: it doubles the epoch length from 30,000 blocks to 60,000 blocks. This effectively halves the rate at which the DAG file grows, immediately reducing the current DAG size and extending the useful life of 3 GB and 4 GB GPU miners by approximately three years.

The modified version of Ethash used after the fork is called ETChash. While structurally similar to the original Ethash algorithm, the change in epoch calibration means that ETC mining now follows its own trajectory, separate from Ethereum’s main chain. This gives Ethereum Classic greater independence in managing its mining ecosystem.

Client Support and Network Split

The Thanos upgrade required coordination among several node implementations, and not all clients chose to support the change. OpenEthereum, OpenETC, Classic Geth, and MultiGeth all announced they would discontinue support for the ETC network following the fork. These clients would cease operations on the ETC chain, effectively leaving the upgraded network.

Hyperledger Besu, an enterprise-grade Ethereum client maintained by the Hyperledger Foundation, confirmed it would continue supporting ETC after the fork. Core-Geth also implemented support for ETChash. Node operators were advised to upgrade their software before the fork to ensure seamless operation on the new chain.

The testnet for Thanos had been activated on October 18, giving developers and miners over five weeks to test the upgrade in a live environment before the mainnet deployment.

Implications for Decentralization and ASIC Resistance

The original purpose of the DAG mechanism was to provide ASIC resistance — ensuring that specialized mining hardware could not easily outperform consumer GPUs. By keeping smaller GPUs in the game, the Thanos fork reinforces this original design philosophy. More miners with accessible hardware means greater network decentralization and a more equitable distribution of mining rewards.

The fork also positions ETC as an attractive destination for GPU miners who might eventually be priced out of Ethereum mining as its DAG continues to grow. With Ethereum itself planning a transition to proof-of-stake through Ethereum 2.0 — with the Beacon Chain genesis scheduled for December 1, 2020 — ETC’s commitment to proof-of-work mining gives displaced Ethereum miners a potential new home.

Market Context

Ethereum Classic was trading at approximately $6.32 on November 28, 2020, gaining 1.9% on the day according to Kraken’s market report. While the price action was modest compared to the broader crypto rally — Bitcoin was at $17,725 and Ethereum at $538 — the fundamental upgrade represented long-term value for the network’s mining infrastructure. The broader market was buoyant, with total spot trading volume reaching $468.7 million on Kraken alone, roughly six times normal weekend levels.

Why This Matters

The Thanos hard fork is a pragmatic solution to a structural problem facing Ethereum Classic. By reducing the DAG growth rate, the upgrade ensures that mining remains accessible to a broader range of participants, directly supporting the network’s decentralization. At a time when Ethereum is preparing to move away from proof-of-work entirely, ETC is doubling down on GPU mining as a core value proposition. The decision by multiple client teams to drop ETC support is notable, but the continued backing of Hyperledger Besu and Core-Geth provides sufficient infrastructure for the network to operate securely. For miners looking beyond Ethereum’s transition to proof-of-stake, the Thanos upgrade makes ETC a more viable and sustainable long-term mining option.

Disclaimer: This article is for informational purposes only and does not constitute financial advice. Cryptocurrency investments carry significant risk. Always conduct your own research before making investment decisions.