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  1. Name
  2. Motivation
  3. inotify vs fanotify
  4. Installing
  5. How to use
  6. Example of usage
  7. Other uses
  8. Clustering
  9. Known building issues
  10. Support
  11. Developing

1. Name

Why "clsync"? The first name of the utility was "insync" (due to inotify), but then I suggested to use "fanotify" instead of "inotify" and utility was been renamed to "fasync". After that I started to intensively write the program. However I faced with some problems in "fanotify", so I was have to temporary fallback to "inotify", then I decided, that the best name is "Runtime Sync" or "Live Sync", but "rtsync" is a name of some corporation, and "lsync" is busy by "lsyncd" project (""). So I called it "clsync", that should be interpreted as "lsync, but on c" due to "lsyncd" that written on "LUA" and may be used for the same purposes.

UPD: Also I was have to add somekind of clustering support. It's multicast notifing subsystem to prevent loops on bidirection syncing. So "clsync" also can be interpreted as "cluster live sync". ;)

2. Motivation

This utility was been writted for two purposes:

  • for making failover clusters
  • for making backups of them

To do failover cluster I've tried a lot of different solutions, like "simple rsync by cron", "glusterfs", "ocfs2 over drbd", "common mirrorable external storage", "incron + perl + rsync", "inosync", "lsyncd" and so on. Currently we are using "lsyncd", "ceph" and "ocfs2 over drbd". However all of this solutions doesn't arrange me, so I was have to write own utility for this purpose.

To do backups we also tried a lot of different solution, and again I was have to write own utility for this purpose.

The best known (for me) replacement for this utility is "lsyncd", however:

  • It's code is on LUA. There a lot of problems connected with it, for example:
    • It's more difficult to maintain the code with ordinary sysadmin.
    • It really eats 100% CPU sometimes.
    • It requires LUA libs, that cannot be easily installed to few of our systems.
  • It's a little buggy. That may be easily fixed for our cases, but LUA. :(
  • It doesn't support pthread or something like that. It's necessary to serve huge directories with a lot of containers right.
  • It cannot run rsync for a pack of files. It runs rsync for every event. :(
  • Sometimes, it's too complex in configuration for our situation.
  • It can't set another event-collecting delay for big files. We don't want to sync big files (>1GiB) so often as ordinary files.
  • Shared object (.so file) cannot be used as rsync-wrapper.

Sorry, if I'm wrong. Let me know if it is, please :). "lsyncd" - is really good and useful utility, just it's not appropriate for us.

UPD.: Also clsync was used to replace incron/csync2/etc in HPC-clusters for syncing /etc/{passwd,shadow,group,shells} files.

3. inotify vs fanotify:

It's said, that fanotify is much better, than inotify. So I started to write this program with using of fanotify. However I encountered the problem, that fanotify was unable to catch some important events at the moment of writing the program, like "directory creation" or "file deletion". So I switched to "inotify", leaving the code for "fanotify" in the safety... So, don't use "fanotify" in this utility ;).

4. Installing

Debian/ubuntu-users can try to install it directly with apt-get:

apt-get install clsync

If it's required to install clsync from the source, first of all, you should install dependencies to compile it. On debian-like systems you should execute something like:

apt-get install libglib2.0-dev autoreconf gcc

Next step is generating Makefile. To do that usually it's enought to execute:

autoreconf -i && ./configure

Next step is compiling. To compile usually it's enough to execute:


Next step is installing. To install usually it's enough to execute:

su -c 'make install'

5. How to use

How to use is described in "man" ;). What is not described, you can ask me personally (see "Support").

6. Example of usage

Example of usage, that works on my PC is in directory "examples". Just run "" and try to create/modify/delete files/dirs in "example/testdir/from". All modifications should appear (with some delay) in directory "example/testdir/to" ;)

For dummies:

pushd /tmp
git clone
cd clsync
autoreconf -fi
export PATH_OLD="$PATH"
export PATH="$(pwd):$PATH"
cd examples
export PATH="$PATH_OLD"

Now you can try to make changes in directory "/tmp/clsync/examples/testdir/from" (in another terminal). Wait about 7 seconds after the changes and check directory "/tmp/clsync/examples/testdir/to". To finish the experiment press C (control+c) in clsync's terminal.

cd ../..
rm -rf clsync

Note: There's no need to change PATH's value if clsync is installed system-wide, e.g. with

make install

For dummies, again (with "make install"):

pushd /tmp
git clone
cd clsync
autoreconf -fi
sudo make install
cd examples

Directory "/tmp/clsync/examples/testdir/from" is now synced to "/tmp/clsync/examples/testdir/to" with 7 seconds delay. To terminate the clsync press C (control+c) in clsync's terminal.

cd ..
sudo make uninstall
cd ..
rm -rf clsync

For really dummies or/and lazy users, there's a video demonstration:

7. Other uses

Also, clsync may be used to do nearly atomic directory recursive copy.

For example, command

ionice -c 3 clsync -L /dev/shm/clsync --exit-on-no-events -x 23 -x 24 -M rsyncdirect -S $(which rsync) -W /path/from -D /path/to -d1

may be used to copy "/path/from" into "/path/to" with sync up of changes made (in "/path/from") while the copying. It will copy new changes over and over until there will be no changes, and then clsync will exit.

8. Clustering

I've started to implement support of bi-directional syncing with using multicast notifing of other nodes. However it became a long task, so it was suspended for next releases.

However let's solve next hypothetical problem. For example, you're using LXC and trying to replicate containers between two servers (to make failover and load balancing).

In this case you have to sync containers in both directions. However, if you just run clsync to sync containers to neighboring node on both of them, you'll get sync-loop [file-update on A causes file-update on B causes file-update on A causes ...].

Well, in this case I with my colleagues were using separate directories for every node of cluster (e.g. "/srv/nodes/<NODE NAME>/containers/<CONTAINERS>") and syncing every directory only in one direction. That was failover with load-balancing, but very unconvenient. So I've started to write code for bi-directional syncing, however it's no time to complete it :(. So Andrew Savchenko proposed to run one clsync-instance per container. And this's really good solution. It's just need to start clsync-process when container starts and stop the process when containers stops. The only problem is split-brain, that can be solved two ways:

  • by human every time;
  • by scripts that chooses which variant of container to save.

Example of the script is just a script that calls "find" on both sides to determine which side has the latest changes :)

9. Known building issues

May be problems with "configuring" or compilation. In this case just try next command: echo '#define REVISION "-custom"' > revision.h; gcc -std=gnu99 -D_FORTIFY_SOURCE=2 -DPARANOID -pipe -Wall -ggdb3 --param ssp-buffer-size=4 -fstack-check -fstack-protector-all -Xlinker -zrelro -pthread $(pkg-config --cflags glib-2.0) $(pkg-config --libs glib-2.0) -ldl *.c -o /tmp/clsync

10. Support

To get support, you can contact with me this ways:

  • Official IRC channel of "clsync":
  • Where else can you find me: IRC:SSL+UTF-8,xaionaro,xai
  • And e-mail:,; PGP pubkey: 0x8E30679C

11. Developing

I started to write "DEVELOPING" and "PROTOCOL" files. You can look there if you wish. ;)

I'll be glad to receive code contribution :)

                                           -- Dmitry Yu Okunev <> 0x8E30679C