Blockchain sync lost after restarting geth?

Question

I'm new to mining ethereum so this may be a very basic question.

I ran geth in attempt to sync the blockchain on my machine as follows

geth --rpc --fast --cache=2048

After letting it spin for about a day I ran get attach and saw the following

> web3.eth.syncing
{
  currentBlock: 3274586,
  highestBlock: 3275837,
  ...
}

I then restarted geth, with the intention of not passing in the fast flag and allowing geth to continue syncing the blockchain normally so that I could start to mine. However when running geth attach and checking the value of web3.eth.syncing I saw the the following

{
  currentBlock: 580,
  highestBlock: 1707213,
  ...
}

Is there some way to start geth with the blockchain from my initial sync, or do I have a misunderstanding of how geth is meant to be used?

Answer 1

Because geth --fast will not download the entire blocks

Instead of processing the entire Blockchain one link at a time, and replay all transactions that ever happened in history, fast syncing downloads the transaction receipts along the blocks, and pulls an entire recent state database.

When you remove --fast flag, the chain will start syncing all the blocks from very beginning.