Block devices synchronization tool
Fetching latest commit…
Cannot retrieve the latest commit at this time.
Failed to load latest commit information.


This Linux-specific tool efficiently copies contents of one block device to another, by reading devices block-by-block and only writing to second device blocks with non-matching content.

It is more efficient than plain dd(1) if the following conditions are met:

  • infrequent writes to the second device do not ruin speed of linear read,
  • second device write speed is way slower than read speed.

Otherwise don't bother, and just use dd(1).

Note: this tool does not do any locking or thawing of filesystems, so copy might not be an exact snapshot of source device at any moment of time. IOW, use it with caution, during low-activity time, and on journalled filesystems.


Implementation of JWZ backups scheme for laptop with complex structure of LVM volumes and USB-attached hard drive.

License and Copyright


© 2009-2011 Mikhail Gusarov