How to Achieve Real-Time Data Sync with rsync, inotify, and sersync on CentOS

This article explains how to use rsync together with inotify‑tools or sersync to achieve real‑time data synchronization between two CentOS 7 virtual machines.

1. Create virtual machines and prepare hosts

Two VMs (xy1 and xy2) are created on VMware Workstation (NAT mode) using the CentOS‑7‑x86_64‑DVD‑1708.iso image. Hostnames and IPs are set as follows:

xy1: 192.168.91.131

xy2: 192.168.91.132

Configuration steps include setting the hostname, mounting the ISO, configuring YUM sources, assigning a static IP, and installing vim.

2. Install rsync

(1) Install xinetd and rsync

[root@xy1 ~]# yum install xinetd rsync -y

[root@xy2 ~]# yum install xinetd rsync -y
[root@xy2 ~]# rsync --daemon
[root@xy2 ~]# netstat -an | grep 873

(2) Use rsync to back up data

Backup the web root directory /var/www/html on xy1 to /web_bak on xy2.

Commands:

# create test user on both hosts
[root@xy1 ~]# useradd rget1 && echo "123456" | passwd --stdin rget1
[root@xy2 ~]# useradd rget1 && echo "123456" | passwd --stdin rget1

# set ACL permissions on the source directory
[root@xy1 ~]# mkdir -p /var/www/html/
[root@xy1 ~]# setfacl -R -m user:rget1:rwx /var/www/html/
[root@xy1 ~]# setfacl -R -m default:rget1:rwx /var/www/html/

# copy test data
[root@xy1 ~]# cp -r /boot/* /var/www/html/

# create destination directory and set ownership
[root@xy2 ~]# mkdir /web_bak
[root@xy2 ~]# chown rget1:rget1 -R /web_bak/

# perform the rsync transfer
[root@xy1 ~]# rsync -avz /var/www/html/ [email protected]:/web_bak/

(3) Backup with a non‑system user

Create /etc/rsyncd.conf with global and module parameters, then start rsync as a daemon.

# edit /etc/rsyncd.conf (create if missing)
# add a welcome message
[root@xy2 ~]# echo "Welcome to Backup Server" > /etc/rsyncd.motd
# create password file
[root@xy2 ~]# echo "rsyncuser:123456" > /etc/rsync.passwd
[root@xy2 ~]# chmod 600 /etc/rsync.passwd
# start services
[root@xy2 ~]# systemctl start xinetd
[root@xy2 ~]# systemctl enable xinetd
[root@xy2 ~]# rsync --daemon --config=/etc/rsyncd.conf
[root@xy2 ~]# netstat -antup | grep 873

(4) Scripted automatic backup

Create a simple bash script that runs the rsync command and schedule it with cron.

#!/bin/bash
rsync -avz /var/www/html/ [email protected]::wwwroot --password-file=/etc/rsync.passwd

Make it executable and add a cron entry, e.g.:

# method 1
echo "01 3 * sh /root/autobackup.sh &" >> /var/spool/cron/root
# method 2
crontab -e   # then add: 01 3 * sh /root/autobackup.sh &

3. Real‑time sync with rsync + inotify‑tools

Increase inotify limits:

fs.inotify.max_queued_events = 32768
fs.inotify.max_user_instances = 1024
fs.inotify.max_user_watches = 90000000

Install inotify‑tools, then run a watcher that triggers rsync on file events:

# install inotify‑tools
wget https://sourceforge.net/projects/inotify-tools/files/inotify-tools/3.13/inotify-tools-3.13.tar.gz
# compile and install
tar -zxvf inotify-tools-3.13.tar.gz
cd inotify-tools-3.13
./configure && make && make install
# start monitoring and syncing
#!/bin/bash
SRC=/var/www/html/
[email protected]:/web_bak
inotifywait -mrq -e create,move,delete,modify $SRC | while read a b c; do
    rsync -azP --delete $SRC $DST
done

Make the script executable, run it, and add it to /etc/rc.local for boot‑time start.

4. Real‑time sync with rsync + sersync

Sersync is an inotify‑based tool that reports the exact files that changed, allowing rsync to transfer only those files.

Key steps:

Download the binary package (e.g., sersync2.5.4_64bit_binary_stable_final.tar.gz) and extract it to /opt/sersync.

Edit confxml.xml to set the local watch path and remote rsync module:

<sersync>
    <localpath watch="/var/www/html">
        <remote ip="192.168.91.132" name="wwwroot"/>
    </localpath>
    <rsync>
        <commonParams params="-artuz"/>
        <auth start="true" users="rsyncuser" passwordfile="/etc/rsync.passwd"/>
    </rsync>
</sersync>

Start the sersync daemon:

/opt/sersync/sersync2 -d -r -o /opt/sersync/confxml.xml

Now any create, delete, or modify operation in /var/www/html on xy1 is instantly mirrored to /web_bak on xy2.

5. Summary

Rsync is a powerful Linux tool for incremental, compressed, and secure file synchronization. Combined with inotify‑tools it can provide near‑real‑time updates, but it syncs the whole directory each time. Sersync improves efficiency by reporting only the changed files, allowing rsync to transfer a minimal data set. Using scripts, cron jobs, or daemon services, administrators can build automated backup solutions that scale from simple push/pull models to fully automated, real‑time replication across servers.