So far I've gotten these rather cryptic messages while syncing:
What do they mean? What is a block header? What is a block receipt? And what is a state entry?
this seems to be a repeating question, I was also trying to find answers to it. Everytime sync is taking forever, these type of questions come up. Generally, an older PC with Dual Core seems to be insufficient, and less of 4Gig RAM seems to always "hang" in importing states. Some references here:
geth: chaindata copied, sync'd, keystore, account updated, balance 0 - heavy activity (disk I/O)
and stepped in here: geth sync status is not clear
and here: https://github.com/ethereum/go-ethereum/issues/14571
I couldn't get near enough to the devs, and documentation is rare. So "code is the doc" seems to be the approach :-)
Form ethereum yellow paper (page 5):
The block in Ethereum is the collection of relevant pieces of information (known as the block header), H, together with information corresponding to the comprised transactions, T, and a set of other block headers U that are known to have a parent equal to the present block’s parent’s parent (such blocks are known as ommers). The block header contains several pieces of information:
... list of header fields
The other two components in the block are simply a list of ommer block headers (of the same format as above) and a series of the transactions.
Also from wiki:
Every block header in the Ethereum blockchain contains pointers to three tries: the state trie, representing the entire state after accessing the block, the transaction trie, representing all transactions in the block keyed by index (ie. key 0: the first transaction to execute, key 1: the second transaction, etc), and the receipt tree, representing the "receipts" corresponding to each transaction. A receipt for a transaction is an RLP-encoded data structure:
[ medstate, gas_used, logbloom, logs ]
So that is what geth imports: it imports blockchain blocks and each block contains header, receipts and state
