syncer -- fast stateful file/disk data syncer.

WARNING: I advise you to use ZFS and its native send/recv commands.
I abandoned that project because of that.

The main purpose of this utility is fast data synchronization between
two hard drives: one is fast (SSD, SATA HDD), another is connected
through slow USB interface. Target is to lower data amounts needed to
be transferred.

This utility is stateful: it keeps precomputed data hashes in separate
statefile and uses it to determine if we need to update block of data.

    # sync from very fast SSD to slow USB connected HDD
    $ syncer -src /dev/ada0 -dst /dev/da0 -state state.bin
    [%%%%%%]
    # all blocks were transferred to da0

Now we have statefile containing cryptographic hashes of the blocks from
source and copy of all read data in destination. Now if we run it again:

    $ syncer -src /dev/ada0 -dst /dev/da0 -state state.bin
    [....%.]
    # only one block was transferred to da0

Only one modified block was transferred during this session. We read all
data from source again, compute hashes and understand what was updated
since the last run. Statefile is updated at the end.

Utility parallelize hash computations among all found CPUs. It updates
statefile atomically (saves data in temporary file and then renames it).
You can configure the blocksize: shorter transfers but bigger statefile
(it is kept in memory), or larger transfer and smaller statefile. All
writes are sequential.

syncer is free software: see the file COPYING for copying conditions.