Managing 500 GB of data with the proposed solution is feasible, but there are a few important factors to consider based on the size of the data and the tools involved.

Key Considerations for 500 GB of Data:

1. File Syncing (Websites, Mail, etc.):
   • rsync or Lsyncd can handle 500 GB, but the initial sync could take some time depending on the speed of your network and servers. After the initial sync, both tools work incrementally, transferring only the changed files, which will greatly reduce the amount of data being transferred regularly.
   • Bandwidth and Latency: If your servers are geographically distant, syncing 500 GB could cause delays. You’ll need a reliable and fast network connection between your servers to avoid long sync times.
   • Storage I/O Performance: Both servers need sufficient disk read/write performance to handle the constant syncing of large amounts of data.
2. Database Replication (MySQL/MariaDB):
   • MySQL replication can handle large databases, but you need to ensure that your database storage and backup strategy is designed to manage the data growth over time.
   • Incremental Changes: As long as the changes to the databases (inserts, updates, etc.) are not extremely large on a day-to-day basis, the replication process should be smooth.
3. GlusterFS for Distributed Storage (if you go this route):
   • Scalability: GlusterFS is designed to handle large datasets, so it can easily support your 500 GB of data. It replicates data across nodes in real time, so you won’t have to worry about manual syncing.
   • Performance: GlusterFS performs best with high-bandwidth and low-latency networks. With 500 GB of data, you’ll need to ensure that the infrastructure (network and disk I/O) is capable of handling the data flow between servers.

Refining the Solution for 500 GB:

1. Initial Sync:
   • If you’re using rsync or Lsyncd, the initial sync could take a long time depending on your connection. You might want to:
     • Perform the initial sync during low-traffic hours.
     • Use compression during sync (rsync can compress files while transferring to reduce network load).
     • If both servers are on the same local network during setup, the initial sync will be faster. Then you can move one server to a remote location for redundancy.
2. Storage on Both Servers:
   • Ensure that both servers have enough storage capacity to handle 500 GB, plus additional room for future growth and any snapshots or backups.
   • Consider using ZFS or ext4 with LVM (Logical Volume Manager) for easy management of storage volumes and snapshots.
3. Backup Strategy:
   • Regular backups are important, especially with this amount of data. Automate backups using tools like Duplicity or rsnapshot. Store backups on a separate disk or a cloud service for redundancy.
4. Monitoring and Alerts:
   • Set up monitoring tools like Prometheus or Zabbix to track server health, disk usage, and syncing performance. This will help you identify potential issues early, such as slow syncing or disk space running out.

Solution Breakdown with 500 GB in Mind:

File Syncing (Web + Mail Data):

• Lsyncd will handle this well for real-time file syncing with incremental changes. You’ll need to make sure that the network between servers is fast enough to sync changes efficiently.
• If the 500 GB includes many small files (e.g., mail directories), you might want to optimize rsync/Lsyncd to handle small files efficiently. Consider setting parameters like --inplace or --whole-file depending on the network and file sizes.

Database Syncing:

• MySQL Replication will handle the database part well. For 500 GB, the database size should be broken down into manageable chunks (i.e., tables and databases), and replication should be set up for just the changes.
• You might also want to enable binary logging in MySQL to keep a history of changes, which will help in recovery.

Mail Data Syncing:

• Dovecot can store emails as individual files. Since mailboxes can grow large over time, rsync or Lsyncd will handle the incremental syncing of mail data well.
• Depending on how active your mail clients are (IMAP users, etc.), ensure regular syncs and backup strategies are in place.

High Availability for 500 GB:

• With 500 GB, GlusterFS is a viable option if you want more automated data redundancy with high availability. It provides real-time file replication and is designed to handle larger data sets across multiple servers.
• Ensure your servers have enough disk I/O capacity and network bandwidth to support real-time replication for this amount of data.

Final Recommendation:

For your scale (50 clients, 500 GB of data):

• Use Lsyncd for real-time file syncing (website and mail directories).
• Set up MySQL/MariaDB replication for the databases.
• Ensure both servers have ample storage capacity and disk I/O performance.
• If you’re comfortable with some complexity and want real-time redundancy, GlusterFS could also be a good choice, but Lsyncd + MySQL replication would be simpler to start with.

This solution will be relatively easy to manage, will handle your current data size, and can scale as your needs grow.