Given that you are new to Linux and are planning to host services for about 50 clients using databases, mail, and websites, you’ll want a solution that balances simplicity, reliability, and ease of setup.

Recommended High Availability (HA) Solution for Your Use Case:

Option 1: rsync + Simple Backup for Data Redundancy

• Why: Since you’re managing a smaller-scale hosting operation, rsync combined with regular backups can be a simple and effective way to ensure that your data is synchronized across servers.
• How:
  • Use rsync to periodically copy website files, database backups, and mail data from the primary server to a secondary server.
  • Automate this using cron jobs to run the synchronization at intervals (e.g., every 30 minutes or hourly).
  • This is easy to set up and doesn’t require complex clustering or advanced configuration.
• Advantages:
  • Simple to set up and easy to maintain.
  • Does not require much learning or configuration.
  • Low resource usage.
• Drawbacks: No real-time replication; if a failure happens, you may lose the most recent changes that happened between syncs.

Option 2: Lsyncd (Live Syncing Daemon) for Real-Time Syncing

• Why: If you need near-real-time syncing of files between two servers, Lsyncd can be a great middle-ground solution.
• How:
  • Lsyncd monitors changes on your primary server and uses rsync to push those changes to the backup server in near real-time.
  • This would ensure that your client websites, databases, and emails are synced with minimal delay.
• Advantages:
  • Easier to set up compared to complex HA systems.
  • Provides real-time syncing of file changes.
• Drawbacks: Does not offer database-level synchronization; you’ll need to configure database backups separately.

Option 3: GlusterFS for Simple Distributed Storage

• Why: If you need a more robust system with real-time data replication across multiple servers without manual intervention, GlusterFS is a good choice.
• How:
  • GlusterFS can create a shared storage system across your servers, where data is automatically replicated. This ensures that if one server goes down, another server has all the same data.
  • It’s reliable for hosting files (websites, mail data) and can be used in combination with database replication (e.g., MySQL replication) for the database side.
• Advantages:
  • Real-time redundancy across multiple servers.
  • No need for manual intervention once set up.
• Drawbacks: Slightly more complex to set up and manage for a Linux beginner, though it’s still simpler than Ceph or other enterprise-grade systems.

Mail & Database Consideration:

• Mail: For mail services, you can set up a simple Postfix/Dovecot mail server and sync the mail directories with one of the above methods (rsync or Lsyncd). For redundancy, you may want to look into configuring a secondary mail server as a fallback.
• Databases: For databases (e.g., MySQL/MariaDB), I would recommend setting up MySQL replication where one server acts as the master, and the other acts as the slave. This ensures that all database changes on the master server are mirrored to the slave in real time.

Simplified Stack for Your Hosting Solution:

• Web Server: Nginx or Apache (both easy to set up and manage).
• Database: MariaDB with master-slave replication.
• Mail Server: Postfix + Dovecot for handling mail, with rsync or Lsyncd for syncing mailboxes.
• File Syncing: Lsyncd or GlusterFS to sync website files in real-time.

Why This Setup Works:

• Ease of Setup: Lsyncd and rsync are straightforward tools that work well for syncing files between servers. MySQL replication is easy to configure and provides database redundancy.
• Low Maintenance: You won’t have to manage a full-fledged high-availability cluster, which would be complex for your current experience level.
• Reliability: You get real-time or near-real-time data replication, reducing the risk of data loss.