High Availability and Disaster Recovery (HADR) at the same time
Evidian SafeKit
HADR architecture with a fast network
2 nodes are put in 2 remotes sites for implementing at the same time high availability and disaster recovery (HADR).
HADR solution with a fast nework
A solution without shared disk like Evidian SafeKit is required with 2 nodes replicating data synchronously and in real-time.
Because there is no data loss with synchronous replication, an automatic application failover can be implemented in case of failures.
Why an extended LAN/VLAN?
- Because there is an automatic failover of the virtual IP address with 2 nodes in the same subnet.
- Because a good latency (typically a round-trip of less than 2ms) is required for synchronous replication with no data loss.
- Because a good bandwidth (typically 1 Gb/s or more) is required for resynchronizing data at failback
HADR architecture with a slow network
2 nodes are put in the first site for high availability and a backup solution is implemented for disaster recovery.
HADR solution with a slow network
A high availability solution like Evidian SafeKit is implemented in the first site with synchronous real-time replication and automatic failover.
And a backup solution is implemented for asynchronous replication through the slow network to the disaster recovery site.
Failover to the disaster recovery site
- As there is data loss with a backup solution, the failover is manual and decided by an administrator.
- Backups are restored on servers at the disaster recovery site. Today, virtual machine backups are often implemented to facilitate this restoration (like Veeam).
- Rerouting to the disaster recovery site is made at the DNS level. The rerouting time depends on DNS cache timeout. And this may require restarting clients that perform their DNS resolution only once during their initialization.
High availability does not remove the need for a backup solution
Real-time replication of high availability solutions like SafeKit does not remove the need for a backup solution. Some events are recoverable only with a backup solution. For example, a ransomware encrypting replicated data on the primary server will encrypt data on the secondary with a real-time replication. Only a backup solution with a retention policy can solve a ransomware attack.
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...
Step 1. Real-time replication
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.
Step 2. Automatic failover
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.
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 takes place without disturbing the Windows or Linux 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 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.
More information on power outage and network isolation in a cluster.
Redundancy at the application level
In this type of solution, only application data are replicated. And only the application is restared in case of failure.
With this solution, restart scripts must be written to restart the application.
We deliver application modules to implement redundancy at the application level (like the Windows module provided in the free trial below). They are preconfigured for well known applications and databases. You can customize them with your own services, data to replicate, application checkers. And you can combine application modules to build advanced multi-level architectures.
This solution is platform agnostic and works with applications inside physical machines, virtual machines, in the Cloud. Any hypervisor is supported (VMware, Hyper-V...).
Redundancy at the virtual machine level
In this type of solution, the full Virtual Machine (VM) is replicated (Application + OS). And the full VM is restarted in case of failure.
The advantage is that there is no restart scripts to write per application and no virtual IP address to define. If you do not know how the application works, this is the best solution.
This solution works with Windows/Hyper-V and Linux/KVM but not with VMware. This is an active/active solution with several virtual machines replicated and restarted between two nodes.
- Solution for a new application (no restart script to write): Windows/Hyper-V, Linux/KVM
More comparison between VM HA vs Application HA
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
- For a large volume of data, use external shared storage.
- 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.
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?
- 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.
New application (real-time replication and failover)
New application (network load balancing and failover)
Database (real-time replication and failover)
- Microsoft SQL Server mirror
- PostgreSQL mirror
- MySQL mirror
- Oracle mirror
- MariaDB mirror
- Firebird mirror
Web (network load balancing and failover)
Full VM or container real-time replication and failover
Amazon AWS
Google GCP
Microsoft Azure
Other clouds
Physical security (real-time replication and failover)
Siemens (real-time replication and failover)
VM HA with the SafeKit Hyper-V or KVM module | Application HA with SafeKit application modules |
SafeKit inside 2 hypervisors: replication and failover of full VM | SafeKit inside 2 virtual or physical machines: replication and failover at application level |
Replicates more data (App+OS) | Replicates only application data |
Reboot of VM on hypervisor 2 if hypervisor 1 crashes Recovery time depending on the OS reboot VM checker and failover (Virtual Machine is unresponsive, has crashed, or stopped working) |
Quick recovery time with restart of App on OS2 if crash of server 1 Around 1 mn or less (see RTO/RPO here) Application checker and software failover |
Generic solution for any application / OS | Restart scripts to be written in application modules |
Works with Windows/Hyper-V and Linux/KVM but not with VMware | Platform agnostic, works with physical or virtual machines, cloud infrastructure and any hypervisor including VMware |
SafeKit with the Hyper-V module or the KVM module | Microsoft Hyper-V Cluster & VMware HA |
No shared disk - synchronous real-time replication instead with no data loss | Shared disk and specific extenal bay of disk |
Remote sites = no SAN for replication | Remote sites = replicated bays of disk across a SAN |
No specific IT skill to configure the system (with hyperv.safe and kvm.safe) | Specific IT skills to configure the system |
Note that the Hyper-V/SafeKit and KVM/SafeKit solutions are limited to replication and failover of 32 VMs. | Note that the Hyper-V built-in replication does not qualify as a high availability solution. This is because the replication is asynchronous, which can result in data loss during failures, and it lacks automatic failover and failback capabilities. |
Evidian SafeKit mirror cluster with real-time file replication and failover |
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3 products in 1 More info > |
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Very simple configuration More info > |
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Synchronous replication More info > |
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Fully automated failback More info > |
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Replication of any type of data More info > |
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File replication vs disk replication More info > |
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File replication vs shared disk More info > |
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Remote sites and virtual IP address More info > |
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Quorum and split brain More info > |
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Active/active cluster More info > |
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Uniform high availability solution More info > |
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RTO / RPO More info > |
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Evidian SafeKit farm cluster with load balancing and failover |
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No load balancer or dedicated proxy servers or special multicast Ethernet address More info > |
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All clustering features More info > |
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Remote sites and virtual IP address More info > |
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Uniform high availability solution More info > |
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Software clustering vs hardware clustering More info > |
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Shared nothing vs a shared disk cluster More info > |
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Application High Availability vs Full Virtual Machine High Availability More info > |
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High availability vs fault tolerance More info > |
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Synchronous replication vs asynchronous replication More info > |
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Byte-level file replication vs block-level disk replication More info > |
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Heartbeat, failover and quorum to avoid 2 master nodes More info > |
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Virtual IP address primary/secondary, network load balancing, failover More info > |
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To kickstart your journey with SafeKit, begin with the quick installation guides.
Packages (8.2)
- Windows (with Microsoft Visual C++ Redistributable)
- Windows (without Microsoft Visual C++ Redistributable)
- Linux
- Supported OS and last fixes
- Previous 7.5