  As ethereum blockchain has become popular, the number of users and
transactions has skyrocketed, causing an explosive increase of its data size.
As a result, ordinary clients using PCs or smartphones cannot easily bootstrap
as a full node, but rely on other full nodes such as the miners to run or
verify transactions. This may affect the security of ethereum, so light
bootstrapping techniques such as fast sync has been proposed to download only
parts of full data, yet the space overhead is still too high. One of the
biggest space overhead that cannot easily be reduced is caused by saving the
state of all accounts in the block's state trie. Fortunately, we found that
more than 90% of accounts are inactive and old transactions are hard to be
manipulated. Based on these observations, this paper propose a novel
optimization technique called ethanos that can reduce bootstrapping cost by
sweeping inactive accounts periodically and by not downloading old
transactions. If an inactive account becomes active, ethanos restore its state
by running a restoration transaction. Also, ethanos gives incentives for
archive nodes to maintain the old transactions for possible re-verification. We
implemented ethanos by instrumenting the go-ethereum (geth) client and
evaluated with the real 113 million transactions from 14 million accounts
between 7M-th and 8M-th blocks in ethereum. Our experimental result shows that
ethanos can reduce the size of the account state by half, which, if combined
with removing old transactions, may reduce the storage size for bootstrapping
to around 1GB. This would be reasonable enough for ordinary clients to
bootstrap on their personal devices.
