DAG stands for Directed Acyclic Graph. In Ethereum, a DAG is created every epoch using a version of the Dagger-Hashimoto Algorithm combining Vitalik Buterin's Dagger algorithm and Thaddeus Dryja's Hashimoto algorithm.
There are quite a few places where the DAG is defined in the docs and literature. These are collated below:
From the yellow paper:
...d being the current
DAG, a large data set needed to compute the mix-hash...
From Wikipedia:
Directed Acyclic Graph:
image credit David Eppstein
In mathematics and computer science, a directed acyclic graph (DAG), is a finite directed graph with no directed cycles.
That is, it consists of finitely many vertices and edges, with each
edge directed from one vertex to another, such that there is no way to
start at any vertex v and follow a consistently-directed sequence of
edges that eventually loops back to v again. Equivalently, a DAG is a
directed graph that has a topological ordering, a sequence of the
vertices such that every edge is directed from earlier to later in the
sequence.
From https://github.com/ethereum/wiki/wiki/Ethash-DAG:
...a great huge dataset known as the DAG...
The Ethash algorithm expects the DAG as a two-dimensional array of
uint32s (4-byte unsigned ints), with dimension (n × 16) where n is a
large number. (n starts at 16777186 and grows from there.) Following
the magic number, the rows of the DAG should be written sequentially
into the file, with no delimiter between rows and each unint32 encoded
in little-endian format.
From Vitalik Buterin's (I think) Dagger Paper, Dec 2013:
Dagger, a memory-hard proof of work based on moderately connected
directed acyclic graphs (DAGs, hence the name), which, while far from
optimal, has much stronger memory-hardness properties than anything
else in use today.
Essentially, the Dagger algorithm works by creating a directed acyclic
graph (the technical term for a tree where each node is allowed to
have multiple parents) with ten levels including the root and a total
of 2^25 - 1 values.
From https://github.com/ethereum/wiki/wiki/Mining#so-what-is-mining-anyway:
...calculating the PoW (Proof of Work) requires subsets of a fixed resource dependent on the nonce and block header.
This resource (a few gigabyte size data) is called a DAG. The DAG is
totally different every 30000 blocks (a 100 hour window, called an
epoch) and takes a while to generate.
From https://github.com/ethereum/wiki/wiki/Mining#ethash-dag
a DAG (directed acyclic graph) for the proof of work algorithm
From https://github.com/ethereum/wiki/wiki/Mining#the-algorithm
a large, transient, randomly generated dataset
From https://github.com/ethereum/wiki/wiki/Ethash
The DAG is the "dataset" in this description of the Ethash algorithm, emphasis mine:
- There exists a seed which can be computed for each block by scanning through the block headers up until that point.
- From the seed, one can compute a 16 MB pseudorandom cache. Light clients store the cache.
- From the cache, we can generate a 1 GB dataset, with the property that each item in the dataset depends on only a small number of items from the cache. Full clients and miners store the dataset. The dataset grows linearly with time.
- Mining involves grabbing random slices of the dataset and hashing them together. Verification can be done with low memory by using the cache to regenerate the specific pieces of the dataset that you need, so you only need to store the cache.
Best Answer
Speeding up
There is no way you can speed up the DAG creation except:
What if you could optimise,
Well, you can actually propose it as a Ethereum Improvement Proposal - EIP
What is DAG?
Dagger Hashimoto is a proposed spec for the mining algorithm for Ethereum 1.0. Dagger Hashimoto aims to simultaneously satisfy two goals:
That being said, the main purpose of DAG is to verify the work; so called PoW(Proof of Work). Means other nodes verifies your block with the PoW that you have done that is the DAG.
The 1 GB DAG size was chosen in order to require a level of memory larger than the size at which most specialized memories and caches are built, but still small enough for ordinary computers to be able to mine with it. For more info, check Ethash Design Rationale.