Rolf Fokkens, july 15th 2013


The lastest (work in progress) version of bdsync can now be downloaded from GitHub.


bdsync — a fast block device synchronizing tool


Bdsync can be downloaded both as tgz and src.rpm. The software is written and tested on Fedora Linux, I expect some Linux specifics in the code with respect to 64 bit lseek (and off64_t).


The current state of the software is "Works for me". If it works for you as well: enjoy!.
The roadmap is currently: fast replacement by another tool, e.g. rsync.


Client: bdsync [--verbose] REMSHCMD LOCDEV REMDEV
Server: bdsync --server [--verbose]
Patch: bdsync --patch [--verbose] [DSTDEV]


Bdsync can be used to synchronize block devices over a network. It generates a "binary diff" in an efficient way by comparing MD5 checksums of 32k blocks of the local block device LOCDEV and the remote block device REMDEV.

This binary diff can be sent to the remote machine and applied to its block device DSTDEV, after which the local blockdev LOCDEV and the remote block device REMDEV are synchronized.

bdsync was built to do the only thing rsync isn’t able to do: synchronize block devices.


Bdsync assumes a client is connecting to a server. The connection isn’t established by the client itself, but by a remote-shell-command REMSHCMD. This REMSHCMD can be any kind of command to make a connection: rsh, ssh, netcat.. If using rsh or ssh REMSHCMD should be the full command to make the connection including the remote bdsync command te be executed in server mode. If using netcat to make the connection, the remote server mode bdsync command should be executed by inetd, xinetd etc.

The --verbose option results in verbose output. In Server mode the output will be sent to syslog. The --verbose option can be repeated raising the verbosity level.

Bdsync can be initiated like this in its most simple form:

bdsync "bdsync -s" /dev/LOCDEV /dev/REMDEV > DEV.bdsync

This generates a diff DEV.rsync of the /dev/LOCDEV and the /dev/REMDEV devices which both are local. A more realistic example is this:

bdsync "ssh bdsync --server" /dev/LOCDEV /dev/REMDEV | gzip > DEV.bdsync.gz

This bdsync client makes an ssh connection to as user doe and executes a bdsync client by passing it the --server option. The generated diff output is passed on to gzip which creates a compressed diff file DEV.bdsync.gz.

On the remote machine the user doe can apply the patch by executing:

gzip < DEV.bdsync.gz | bdsync --patch /dev/DSTDEV

The reason to use a binary patch file instead of instantly patching the remote block device REMDEV is twofold:


Sending over a complete diff allows to synchronize in a consistent way in case of an interruption (powerloss, network malfunction) since you can choose to apply the (complete) diff or not.


Compression of the diff can easily be done, without introducing complexity in bdsync itself.