Lsyncd monitors a local filesystem for changes and mirrors those changes to a filesystem on a remote server. The advantage of lsyncd over rsync is the former’s ability to detect filesystem changes without having to re-scan the entire source and target directory structures. This makes lsyncd faster and more efficient. And you can still use rsync with lsyncd to handle the actual data transfer.

Here’s a quick example of installing and using lsyncd. In this example the source_server and target_server are both RHEL 6.4 64-bit VMs running on ESX host. To make things simple for myself, I was using a root account. However, this is not recommended, unless the filesystems you are synchronizing require root access.

Configure key-based SSH authentication

If using the root account, you must allow direct root SSH on the target server.

Edit target_server:/etc/ssh/sshd_config and set:
PermitRootLogin yes

Save the file and restart SSH:
service sshd restart

Generate SSH key-pair for your account and send the public key to the target_server.

on the source_server:

ssh-keygen -t rsa
ssh-copy-id target_server

test ssh connection:

ssh -qt target_server "hostname"

Install Lua and Lsyncd

yum -y install lua*
yum -y install lsyncd

NOTE: the version of lsyncd currently (Nov 2014) available from RHEL/CentOS repos is 2.1.4. Version 2.1.5 brings back a potentially very valuable functionality of initiating file sync after file attribute change (i.e. file ownership, permissions, etc). If that’s something you may need, here’s how to get lsyncd 2.1.5

cd /tmp
wget http://mirrors.htbindustries.org/CentOS/6/x86_64/lsyncd-2.1.5-1.el6.x86_64.rpm
yum -y install /tmp/lsyncd-2.1.5-1.el6.x86_64.rpm

Before continuing, this would be a good time to do a quick test.

on source_server generate some folders and file for testing:

mkdir /tmp/test1
for i in `seq -w 1 10` ; do mkdir /tmp/test1/dir${i} ; for j in `seq -w 1 10` ; do touch /tmp/test1/dir${i}/file${j} ; done ; done

on the target_server create the base folder:

mkdir /tmp/test1

from source_server run the sync:

lsyncd -rsyncssh /tmp/test1/ root@target_server /tmp/test1/

The next step is to create a configuration file and run lsyncd as a service. I would recommend removing the default configuration file – it’s causes problems with the /etc/init.d/lsyncd start-up script:

rm /etc/sysconfig/lsyncd

Here’s an example configuration file (/etc/lsyncd.conf) for the example above:

settings {
        logfile = "/var/log/lsyncd/lsyncd.log",
        statusFile = "/var/log/lsyncd/lsyncd.status",
        nodaemon = false,
        maxDelays = 3,
        onStartup = true,
        onAttrib = true,
        onCreate = true,
        onDelete = true,
        onModify = true,
        onMove = true,
        delete = true,
}

sync {
        default.rsyncssh,
        source = "/tmp/test1",
        host = "target_server",
        targetdir = "/tmp/test1",
        delay=3,
        rsync = {
                        owner = true,
                        perms = true,
                        checksum = true,
                        compress = false,
                        acls = true,
                        verbose = true,
                        _extra = {"-aKx"},
        }
}

The final steps are to create the log directory and activate the lsyncd service.

mkdir -p /var/log/lsyncd
chkconfig lsyncd on
service lsyncd start
service lsyncd status

Lsyncd can be a bit fiddly to configure, but it is well worth the effort. Hopefully, these instructions will make it a bit easier for you.