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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

Overview

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

Product
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
Failover
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.

Failback
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.

Comparison of SafeKit with Traditional High Availability (HA) Clusters

How does SafeKit compare to traditional High Availability (HA) cluster solutions?

This comparison highlights the fundamental differences between SafeKit and traditional High Availability (HA) cluster solutions like Failover Clusters, Virtualization HA, and SQL Always-On. SafeKit is designed as a low-complexity, software-only solution for generic application redundancy, contrasting with the high complexity and specific storage requirements (shared storage, SAN) typical of traditional HA mechanisms.
Comparison of SafeKit with traditional High Availability (HA) clusters
Solutions Complexity Comments
Failover Cluster (Microsoft) High Specific Storage (shared storage, SAN)
Virtualization (VMware HA) High Specific Storage (shared storage, SAN, vSAN)
SQL Always-On (Microsoft) High Only SQL is redundant, requires SQL Enterprise Edition
Evidian SafeKit Low Simplest, generic and software-only. Unsuitable for large data replication.

SafeKit's Advantage in Application Redundancy

SafeKit achieves its low-complexity High Availability through a simple, software-based mirroring mechanism that eliminates the need for expensive, dedicated hardware like a SAN (Storage Area Network). This makes it a highly accessible solution for quickly implementing application redundancy without complex infrastructure changes.

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.

Alternative

Why a replication < 1,000,000 files?

  • 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.

Why a failover ≤ 32 replicated VMs?

  • 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.

Why a LAN/VLAN network between remote sites?

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.

SafeKit High Availability (HA) Solutions: Quick Installation Guides for Windows and Linux Clusters

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.

SafeKit High Availability (HA) Solutions: Quick Installation Guides (with downloadable .safe modules)
Application Category HA Scenario (High Availability) Technology / Product .safe Module Installation Guide
New Applications Real-Time Replication and Failover Windows mirror.safe View Guide: Windows Replication
New Applications Real-Time Replication and Failover Linux mirror.safe View Guide: Linux Replication
New Applications Network Load Balancing and Failover Windows farm.safe View Guide: Windows Load Balancing
New Applications Network Load Balancing and Failover Linux farm.safe View Guide: Linux Load Balancing
Databases Replication and Failover Microsoft SQL Server sqlserver.safe View Guide: SQL Server Cluster
Databases Replication and Failover PostgreSQL postgresql.safe View Guide: PostgreSQL Replication
Databases Replication and Failover MySQL mysql.safe View Guide: MySQL Cluster
Databases Replication and Failover Oracle oracle.safe View Guide: Oracle Failover Cluster
Databases Replication and Failover Firebird firebird.safe View Guide: Firebird HA
Web Servers Load Balancing and Failover Apache apache_farm.safe View Guide: Apache Load Balancing
Web Servers Load Balancing and Failover IIS iis_farm.safe View Guide: IIS Load Balancing
Web Servers Load Balancing and Failover NGINX farm.safe View Guide: NGINX Load Balancing
VMs and Containers Replication and Failover Hyper-V hyperv.safe View Guide: Hyper-V VM Replication
VMs and Containers Replication and Failover KVM kvm.safe View Guide: KVM VM Replication
VMs and Containers Replication and Failover Docker mirror.safe View Guide: Docker Container Failover
VMs and Containers Replication and Failover Podman mirror.safe View Guide: Podman Container Failover
VMs and Containers Replication and Failover Kubernetes K3S k3s.safe View Guide: Kubernetes K3S Replication
AWS Cloud Real-Time Replication and Failover AWS mirror.safe View Guide: AWS Replication Cluster
AWS Cloud Network Load Balancing and Failover AWS farm.safe View Guide: AWS Load Balancing Cluster
GCP Cloud Real-Time Replication and Failover GCP mirror.safe View Guide: GCP Replication Cluster
GCP Cloud Network Load Balancing and Failover GCP farm.safe View Guide: GCP Load Balancing Cluster
Azure Cloud Real-Time Replication and Failover Azure mirror.safe View Guide: Azure Replication Cluster
Azure Cloud Network Load Balancing and Failover Azure farm.safe View Guide: Azure Load Balancing Cluster
Physical Security / VMS Real-Time Replication and Failover Milestone XProtect milestone.safe View Guide: Milestone XProtect Failover
Physical Security / VMS Real-Time Replication and Failover Nedap AEOS nedap.safe View Guide: Nedap AEOS Failover
Physical Security / VMS Real-Time Replication and Failover Genetec (SQL Server) sqlserver.safe View Guide: Genetec SQL Failover
Physical Security / VMS Real-Time Replication and Failover Bosch AMS (Hyper-V) hyperv.safe View Guide: Bosch AMS Hyper-V Failover
Physical Security / VMS Real-Time Replication and Failover Bosch BIS (Hyper-V) hyperv.safe View Guide: Bosch BIS Hyper-V Failover
Physical Security / VMS Real-Time Replication and Failover Bosch BVMS (Hyper-V) hyperv.safe View Guide: Bosch BVMS Hyper-V Failover
Physical Security / VMS Real-Time Replication and Failover Hanwha Vision (Hyper-V) hyperv.safe View Guide: Hanwha Vision Hyper-V Failover
Physical Security / VMS Real-Time Replication and Failover Hanwha Wisenet (Hyper-V) hyperv.safe View Guide: Hanwha Wisenet Hyper-V Failover
Siemens Products Real-Time Replication and Failover Siemens Siveillance suite (Hyper-V) hyperv.safe View Guide: Siemens Siveillance HA
Siemens Products Real-Time Replication and Failover Siemens Desigo CC (Hyper-V) hyperv.safe View Guide: Siemens Desigo CC HA
Siemens Products Real-Time Replication and Failover Siemens Siveillance VMS SiveillanceVMS.safe View Guide: Siemens Siveillance VMS HA
Siemens Products Real-Time Replication and Failover Siemens SiPass (Hyper-V) hyperv.safe View Guide: Siemens SiPass HA
Siemens Products Real-Time Replication and Failover Siemens SIPORT (Hyper-V) hyperv.safe View Guide: Siemens SIPORT HA
Siemens Products Real-Time Replication and Failover Siemens SIMATIC PCS 7 (Hyper-V) hyperv.safe View Guide: SIMATIC PCS 7 HA
Siemens Products Real-Time Replication and Failover Siemens SIMATIC WinCC (Hyper-V) hyperv.safe View Guide: SIMATIC WinCC HA