SAN vs NAS shared storage for a failover cluster

Server 1 (PRIM) runs the Windows or Linux application. Clients are connected to a virtual IP address. SafeKit replicates in real time modifications made inside files through the network.

The replication is synchronous with no data loss on failure contrary to asynchronous replication.
You just have to configure the names of directories to replicate in SafeKit. There are no pre-requisites on disk organization. Directories may be located in the system disk.

When Server 1 fails, Server 2 takes over. SafeKit switches the virtual IP address and restarts the Windows or Linux application automatically on Server 2.
The application finds the files replicated by SafeKit uptodate on Server 2. The application continues to run on Server 2 by locally modifying its files that are no longer replicated to Server 1.

The failover time is equal to the fault-detection time (30 seconds by default) plus the application start-up time.

Failback involves restarting Server 1 after fixing the problem that caused it to fail.
SafeKit automatically resynchronizes the files, updating only the files modified on Server 2 while Server 1 was halted.

Failback takes place without disturbing the Windows or Linux application, which can continue running on Server 2.

After reintegration, the files are once again in mirror mode, as in step 1. The system is back in high-availability mode, with the Windows or Linux application running on Server 2 and SafeKit replicating file updates to Server 1.

If the administrator wishes the application to run on Server 1, he/she can execute a "swap" command either manually at an appropriate time, or automatically through configuration.

In this solution, only the application data are replicated. In the event of a failure, only the application is restarted, not the entire operating system or VM.

In this solution, the full Virtual Machine (VM) is replicated, including both the Application and the Operating System (OS). In the event of a failure, the full VM is restarted.

Resynchronization time after a failure (step 3)

• 1 Gb/s network ≈ 3 Hours for 1 Tera-bytes.
• 10 Gb/s network ≈ 1 Hour for 1 Tera-bytes or less depending on disk write performances.

Alternative

• For a large volume of data, use external shared storage.
• More expensive, more complex.

• Resynchronization time performance after a failure (step 3).
• Time to check each file between both nodes.

Alternative

• Put the many files to replicate in a virtual hard disk / virtual machine.
• Only the files representing the virtual hard disk / virtual machine will be replicated and resynchronized in this case.

• Each VM runs in an independent mirror module.
• Maximum of 32 mirror modules running on the same cluster.

Alternative

• Use an external shared storage and another VM clustering solution.
• More expensive, more complex.

• Automatic failover of the virtual IP address with 2 nodes in the same subnet.
• Good bandwidth for resynchronization (step 3) and good latency for synchronous replication (typically a round-trip of less than 2ms).

Alternative

• Use a load balancer for the virtual IP address if the 2 nodes are in 2 subnets (supported by SafeKit, especially in the cloud).
• Use backup solutions with asynchronous replication for high latency network.

This table presents the SafeKit High Availability (HA) solutions, categorized by application and operating environment (Databases, Web Servers, VMs, Cloud). Identify the specific pre‑configured .safe module (e.g., mirror.safe, farm.safe, and others) required for real‑time replication, load balancing, and automatic failover of critical business applications on Windows or Linux. Simplify your HA cluster setup with direct links to quick installation guides, each including a download link for the corresponding .safe module.

A SafeKit .safe module is essentially a pre‑configured High Availability (HA) template that defines how a specific application will be clustered and protected by the SafeKit software. In practice, it contains a configuration file (userconfig.xml) and restart scripts.