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Byte-level file replication vs block-level disk replication

Evidian SafeKit

Byte-level file replication vs block-level disk replication in a high availability cluster


This article explores the pros and cons of byte-level file replication vs block-level disk replication in a high availability cluster. We are looking at volume of replicated data, impact on application data organization, recovery time, simplicity of implementation.

Byte-level file replication vs block-level disk replication

The following comparative tables explain in detail the byte-level file replication implemented by SafeKit, a high availability software product.

What is byte-level file replication?

Byte-level file replication (like with SafeKit) means that only modifications inside files are replicated.

Synchronous replication is required in a high vailability cluster for having 0 data loss in case of failure. Asynchronous replication is for backup solutions.

The volume of replicated data is reduced to information modified by applications inside their files. No extra data is replicated.

There is no impact on data organization for an application. For instance, if an application has its data in the system disk, byte-level file replication is working.

Recovery time (RTO) in the event of a failover is reduced to the application restart time on the secondary server's replicated files.

Finally, the solution is very simple to configure as only the paths of directories to replicate are configured.

What is block-level disk replication?

Block-level disk replication (like with DRBD) means that only modifications inside a disk are replicated.

The volume of replicated data is not reduced to information modified by applications. Extra data are replicated like the meta data for managing the disk (list of free blocks, file system internal information).

There is a strong impact on the organization of application data. All data must be localized in the replicated disk. At least, it requires an application reconfiguration. Or, it is impossible if some data to replicate are in the system disk, because this disk must remain specific to each server.

The recovery time (RTO) increases with the file system recovery procedure on the replicated disk after a failover.

Finally, the solution is not easy to configure because skills are required to configure a special disk with a file system. Additionally, application skills are required to configure application data in the replicated disk.

Pros and cons 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 as DRBD
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.

Replicates 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 High Availability Differentiators against Competition

Partners, the success with SafeKit

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

With many references in many countries won by partners, SafeKit has proven to be the easiest solution to implement for redundancy and high availability of building management, video management, access control, SCADA software...

Building Management Software (BMS)

Video Management Software (VMS)

Electronic Access Control Software (EACS)

SCADA Software (Industry)

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.


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 ≤ 32 replicated VMs?

  • Each VM runs in an independent mirror module.
  • Maximum of 32 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 Modules for Plug&Play Redundancy and High Availability Solutions

Advanced clustering architectures

Several modules can be deployed on the same cluster. Thus, advanced clustering architectures can be implemented:

Evidian SafeKit Webinar

Evidian SafeKit Overview Slides

  • Demonstration
  • Examples of redundancy and high availability solution
  • Evidian SafeKit sold in many different countries with Milestone
  • 2 solutions: virtual machine cluster or application cluster
  • Distinctive advantages
  • More information on the web site

More slides

Evidian SafeKit 8.2

All new features compared to SafeKit 7.5 described in the release notes


One-month license key

Technical documentation


Modules and quick installation

SafeKit 8.2 Training


  1. Overview / pptx

    • Demonstration
    • Examples of redundancy and high availability solution
    • Evidian SafeKit sold in many different countries with Milestone
    • 2 solutions: virtual machine or application cluster
    • Distinctive advantages
    • More information on the web site
  2. Competition / pptx

    • Cluster of virtual machines
    • Mirror cluster
    • Farm cluster

Installation, Console, CLI

  1. Install and setup / pptx
    • Package installation
    • Nodes setup
    • Upgrade
  2. Web console / pptx
    • Configuration of the cluster
    • Configuration of a new module
    • Advanced usage
    • Securing the web console
  3. Command line / pptx
    • Configure the SafeKit cluster
    • Configure a SafeKit module
    • Control and monitor

Advanced configuration

  1. Mirror module / pptx
    • start_prim / stop_prim scripts
    • userconfig.xml
    • Heartbeat (<hearbeat>)
    • Virtual IP address (<vip>)
    • Real-time file replication (<rfs>)
    • How real-time file replication works?
    • Mirror's states in action
  2. Farm  module / pptx
    • start_both / stop_both scripts
    • userconfig.xml
    • Farm heartbeats (<farm>)
    • Virtual IP address (<vip>)
    • Farm's states in action
  1. Checkers / pptx
    • userconfig.xml
    • errd checker
    • intf and ip checkers
    • custom checker
    • splitbrain checker for a mirror module
    • tcp, ping, module checkers
    • Checkers in action


  1. Troubleshooting / pptx
    • Analyze yourself the logs
    • Take snapshots for support
    • Boot / shutdown
    • Web console / Command lines
    • Mirror / Farm / Checkers
    • Running an application without SafeKit


  1. Evidian support / pptx
    • Get permanent license key
    • Register on support.evidian.com
    • Call desk