Running a Slasher

Lighthouse includes a slasher for identifying slashable offences comitted by other validators and including proof of those offences in blocks.

Running a slasher is a good way to contribute to the health of the network, and doing so can earn extra income for your validators. However it is currently only recommended for expert users because of the immaturity of the slasher UX and the extra resources required.

Minimum System Requirements

  • Quad-core CPU
  • 16 GB RAM
  • 256 GB solid state storage (in addition to space for the beacon node DB)

How to Run

The slasher runs inside the same process as the beacon node, when enabled via the --slasher flag:

lighthouse bn --slasher --debug-level debug

The slasher hooks into Lighthouse's block and attestation processing, and pushes messages into an in-memory queue for regular processing. It will increase the CPU usage of the beacon node because it verifies the signatures of otherwise invalid messages. When a slasher batch update runs, the messages are filtered for relevancy, and all relevant messages are checked for slashings and written to the slasher database.

You should run with debug logs, so that you can see the slasher's internal machinations, and provide logs to the devs should you encounter any bugs.


The slasher has several configuration options that control its functioning.

Database Directory

  • Flag: --slasher-dir PATH
  • Argument: path to directory

By default the slasher stores data in the slasher_db directory inside the beacon node's datadir, e.g. ~/.lighthouse/{network}/beacon/slasher_db. You can use this flag to change that storage directory.

History Length

  • Flag: --slasher-history-length EPOCHS
  • Argument: number of epochs
  • Default: 4096 epochs

The slasher stores data for the history-length most recent epochs. By default the history length is set high in order to catch all validator misbehaviour since the last weak subjectivity checkpoint. If you would like to reduce the resource requirements (particularly disk space), set the history length to a lower value, although a lower history length may prevent your slasher from finding some slashings.

Note: See the --slasher-max-db-size section below to ensure that your disk space savings are applied. The history length must be a multiple of the chunk size (default 16), and cannot be changed after initialization.

Max Database Size

  • Flag: --slasher-max-db-size GIGABYTES
  • Argument: maximum size of the database in gigabytes
  • Default: 256 GB

The slasher uses LMDB as its backing store, and LMDB will consume up to the maximum amount of disk space allocated to it. By default the limit is set to accomodate the default history length and around 150K validators but you can set it lower if running with a reduced history length. The space required scales approximately linearly in validator count and history length, i.e. if you halve either you can halve the space required.

If you want a better estimate you can use this formula:

360 * V * N + (16 * V * N)/(C * K) + 15000 * N


  • V is the validator count
  • N is the history length
  • C is the chunk size
  • K is the validator chunk size

Update Period

  • Flag: --slasher-update-period SECONDS
  • Argument: number of seconds
  • Default: 12 seconds

Set the length of the time interval between each slasher batch update. You can check if your slasher is keeping up with its update period by looking for a log message like this:

DEBG Completed slasher update num_blocks: 1, num_attestations: 279, time_taken: 1821ms, epoch: 20889, service: slasher

If the time_taken is substantially longer than the update period then it indicates your machine is struggling under the load, and you should consider increasing the update period or lowering the resource requirements by tweaking the history length.

Chunk Size and Validator Chunk Size

  • Flags: --slasher-chunk-size EPOCHS, --slasher-validator-chunk-size NUM_VALIDATORS
  • Arguments: number of ecochs, number of validators
  • Defaults: 16, 256

Adjusting these parameter should only be done in conjunction with reading in detail about how the slasher works, and/or reading the source code.

Short-Range Example

If you would like to run a lightweight slasher that just checks blocks and attestations within the last day or so, you can use this combination of arguments:

lighthouse bn --slasher --slasher-history-length 256 --slasher-max-db-size 16 --debug-level debug

Stability Warning

The slasher code is still quite new, so we may update the schema of the slasher database in a backwards-incompatible way which will require re-initialization.