Byte-level file replication vs block-level disk replication

Evidian SafeKit

Comparison between byte-level file replication vs block-level disk replication

The deployment of a cluster with byte-level file replication as implemented by SafeKit is much more simple than a cluster with block-level disk replication. The following table explains why.

Simplicity of byte-level file replication vs block-level disk replication

Cluster with byte-level file replication

Cluster with byte-level file replication

Cluster with block-level disk replication

Cluster with block-level disk replication


SafeKit on Windows and Linux

Disks replication products

Application data organization

0 impact on application data organization with SafeKit.

Just define directories to replicate in real-time.

Even directories inside the system disk can be replicated.

Impact on application data organization.

Special configuration of the application to put its data in a replicated disk.

Data in the system disk cannot be replicated.

Data replication

Synchronous byte-level file replication.

Replicate file modification operations generated by application activity

No meta data are replicated. Only data modified in files are replicated, not entire files (byte-level file replication).

Synchronous replication to avoid data loss on failure.

Replicate all data modified inside a replicated disk.

Application data plus meta data are replicated.

For instance, last access time on a file is replicated (last access time is modified each time the file is read).

Complexity of deployment

No - install a software on 2 servers

Yes - require specific IT skills to configure OS and replicated disk


Just restart the application on the second server.

Remount the file system on the replicated disk.

Pass the recovery procedure on the file system.

And then restart the application.


Automatic failback.

Resynchronization of data on the secondary server without stopping the application on the primary server.

No application failover while data are not resynchronized.

All products are not at the same level of features.

Quorum and split brain

Application executed on a single server after a network isolation (split brain).

Coherency of data after a split brain.

No need for a third machine or a quorum disk or a special heartbeat line for split brain.

More information on heartbeat, failover and quorum

Require a special quorum disk or a third quorum server to manage split brain.

Suited for

Software editors which want to add a simple high availability option to their application

Enterprise with IT skills in clustering.

SafeKit: an ideal solution for a partner application

This platform agnostic solution is ideal for a partner with a critical application and who wants to provide a high availability option easy to deploy to many customers.

This clustering solution is also recognized as the simplest to implement by our partners.

How the SafeKit mirror cluster works?

Step 1. Real-time replication

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

File replication at byte level in a mirror cluster

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.

Step 2. Automatic failover

When Server 1 fails, Server 2 takes over. SafeKit switches the virtual IP address and restarts the 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.

Failover in a mirror cluster

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

Step 3. Automatic failback

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 in a mirror cluster

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

Step 4. Back to normal

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

Return to normal operation in a mirror cluster

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.

Typical usage with SafeKit

Why a replication of a few Tera-bytes?

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.


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

Why a replication < 1,000,000 files?

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


  • 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.

Why a failover < 25 replicated VMs?

  • Each VM runs in an independent mirror module.
  • Maximum of 25 mirror modules running on the same cluster.


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

Why a LAN/VLAN network between remote sites?


  • 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.

SafeKit High Availability Differentiators against Competition

Demonstrations of SafeKit High Availability Software

SafeKit Webinar

This webinar presents in 2 minutes Evidian SafeKit.

In this webinar, you will understand SafeKit mirror and farm clusters.

Microsoft SQL Server Cluster

This video shows a mirror module configuration with synchronous real-time replication and failover.

The file replication and the failover are configured for Microsoft SQL Server but it works in the same manner for other databases.

Free trial here

Apache Cluster

This video shows a farm module configuration with load balancing and failover.

The load balancing and the failover are configured for Apache but it works in the same manner for other web services.

Free trial here

Hyper-V Cluster

This video shows a Hyper-V cluster with full replications of virtual machines.

Virtual machines can run on both Hyper-V servers and they are restarted in case of failure.

Free trial here