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I am trying to do a fast sync using geth --fast but I am getting this message:

blockchain not empty, fast sync disabled

How can I empty the blockchain to allow for fast sync?

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4 Answers 4

Reset to default
16

To remove your blockchain with geth, run: geth removedb
Then:

Use --fast but you probably also need --cache=1024 (and --jitvm may also help). Without it, you are running with the default which is --cache=16.

A 50% speed increase is possible just by increasing the cache.

If you are starting from the beginning, use: geth --fast --cache=1024 --jitvm

Depending on your RAM, you can also try with higher values, like --cache=2048.

NOTE: The number after --cache= is the amount of memory allocated to the task of downloading the blockchain!!

Source

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    Note this should now be geth removedb (no dash)
    – Derek
    Commented May 31, 2017 at 13:31
  • Is there a way to do this while geth --fast is already running? I want to leave it running long-term to answer RPC requests but don't want it to eat up my disk.
    – sudo
    Commented Apr 7, 2018 at 20:45
  • I mean a safe way. You can run geth removedb in a separate shell while geth --fast is running. I didn't have any problems doing this then sending a few xacts after, but I don't know if it's safe. Docs say nothing.
    – sudo
    Commented Apr 7, 2018 at 20:56
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The default datadir is:

Mac: ~/Library/Ethereum
Linux: ~/.ethereum
Windows: %APPDATA%/Ethereum

Delete the chaindata subdirectory that you find.

Run geth --fast --cache=1024 for a recommended way to sync the chain quickly (don't forget to use a bigger --cache with a value like 1024, otherwise the default is 16, and it won't sync as quickly).

EDIT: Running geth removedb is the simplest way to delete the blockchain in Geth.

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If you have your blockchain partially or fully imported while starting geth, then using the --fast flag is ignored (the flag is disabled). To use the --fast flag you have to remove all of the data from your chaindata folder.

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    Will starting fresh with --fast take less time than updating from the stored data? At what point would you say it's worth it?
    – TinyTimZamboni
    Commented Jul 18, 2017 at 2:58
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use 

geth --fast --cache=2048

it seems at least Mac does not provide the other options under go1.9.2:

flag provided but not defined: -jitvm
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    This is an old question (March 2016), early iterations of the --fast flag didn't allow restarting geth until it finished syncing, if you did restart it will switch to full mode.
    – Ismael ♦
    Commented Jan 12, 2018 at 3:16

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