Byte-level file replication vs block-level disk replication - Evidian

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Byte-level file replication vs block-level disk replication

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
Architecture	Cluster with byte-level file replication	Cluster with block-level disk replication
Product	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.	Block level disk replication. 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 secondary 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. Automatic failover and automatic failback video	All products are not at the same level of features
Split brain and quorum	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	All products are not at the same level of features
Suited for	Software editors which want to add a simple high availability option to their application	Enterprise with IT skills in clustering

More comparison on high availability solutions

High availability architectures comparison
Feature	SafeKit cluster	Other clusters
Software clustering vs hardware clustering More information...	A simple software cluster with the SafeKit package just installed on two servers	Complex hardware clustering with external storage or network load balancers
Shared nothing vs a shared disk cluster More information...	SafeKit is a shared-nothing cluster: easy to deploy even in remote sites	A shared disk cluster is complex to deploy
Application High Availability vs Full Virtual Machine High Availability More information...	Application HA supports hardware failure and software failure with a quick recovery time (RTO around 1 mn or less). Smooth upgrade of application and OS possible server by server (version N and N+1 can coexist)	Full virtual machines HA supports only hardware failure with a VM reboot and a recovery time depending on the OS reboot. Smooth upgrade not possible
High availability vs fault tolerance	No dedicated server. Each server can be the failover server of the other one. Software failure with restart in another OS environment. Smooth upgrade of application and OS possible server by server (version N and N+1 can coexist)	Secondary server dedicated to the execution of the same application synchronized at the instruction level. Software exception on both servers at the same time. Smooth upgrade not possible
Synchronous replication vs asynchronous replication More information...	SafeKit implements real-time synchronous replication with no data loss in case of failure	With asynchronous replication, there is data loss on failure
Byte-level file replication vs block-level disk replication More information...	SafeKit implements real-time byte-level file replication and is simply configured with application directories to replicate even in the system disk	Block-level disk replication is complex to configure and requires to put application data in a special disk
Heartbeat, failover and quorum to avoid 2 master nodes More information...	To avoid 2 masters, SafeKit proposes a simple split brain checker configured on a router	To avoid 2 masters, other clusters require a complex configuration with a third machine, a special quorum disk, a special interconnect
Virtual IP address primary/secondary, network load balancing, failover More information...	No dedicated proxy servers and no special network configuration are required in a SafeKit cluster for virtual IP addresses	Special network configuration is required in other clusters for virtual IP addresses. Note that SafeKit offers a health check adapted to load balancers

Best practices of a mirror cluster with replication and failover

Evidian SafeKit mirror cluster with real-time file replication and failover
All clustering features		A SafeKit cluster runs on Windows and Linux without the need for expensive shared or replicated disk bays SafeKit includes all clustering features: synchronous real-time file replication, monitoring of server/network/software failures, automatic application restart, virtual IP address switched in case of failure to reroute clients This is not the case with replication-only solutions like replication at the database level which implements only replication The cluster configuration is very simple and made by means of application modules. There is no domain controller or active directory to configure as with Microsoft cluster SafeKit implements quick application restart in case of failure: around 1 mn or less (see RTO/RPO here) Quick application restart is not ensured with full virtual machines replication. In case of hypervisor failure, a full VM must be rebooted on a new hypervisor with a recovery time depending on the OS reboot as with VMware HA or Hyper-V cluster
Synchronous replication		The real-time replication is synchronous with no data loss on failure This is not the case with asynchronous replication
Fully automated failback procedure		After a failure when a server reboots, the replication failback procedure is fully automatic and the failed server reintegrates the cluster without stopping the application on the only remaining server This is not the case with most replication solutions particularly with replication at the database level. Manual operations are required for resynchronizing a failed server. The application may even be stopped on the only remaining server during the resynchonization of the failed server
Replication of any type of data		The replication is working for databases but also for any files which shall be replicated This not the case for replication at the database level
File replication vs disk replication		The replication is based on file directories that can be located anywhere (even in the system disk) This is not the case with disk replication where special application configuration must be made to put the application data in a special disk
File replication vs shared disk		The servers can be put in two remote sites This is not the case with shared disk solutions
Remote sites		All SafeKit clustering features are working for 2 servers in remote sites. Performances of replication depends on the interconnect latency for real-time synchronous replication and on the bandwidth for resynchronizing data on a failed server If both servers are connected to the same IP network through an extended LAN between two remote sites, the virtual IP address of SafeKit is working with rerouting at level 2 If both servers are connected to two different IP networks between two remote sites, the virtual IP address can be configured at the level of a load balancer. SafeKit offers a health check: the load balancer is configured with a URL managed by SafeKit which returns OK on the primary server and NOT FOUND else. This solution is implemented for SafeKit in the Cloud but it can be also implemented with a load balancer on premise
Quorum		With remote sites, the solution works with only 2 servers and for the quorum (network isolation), a simple split brain checker to a router is offered to support a single execution This is not the case for most clustering solutions where a 3^rd server is required for the quorum
Active/active cluster		The secondary server is not dedicated to the restart of the primary server. The cluster can be active-active by running 2 different mirror modules This is not the case with a fault tolerant system where the secondary is dedicated to the execution of the same application synchronized at the instruction level
Uniform high availability solution		SafeKit implements a mirror cluster with replication and failover. But it imlements also a farm cluster with load balancing and failover. Thus a N-tiers architecture can be made highly available and load balanced with the same solution on Windows and Linux (same installation, configuration, administration with the SafeKit console or with the command line interface). This is unique on the market This is not the case with an architecture mixing different technologies for load balancing, replication and failover

Best practices of a farm cluster with load balancing and failover

Evidian SafeKit farm cluster with load balancing and failover
No load balancer or dedicated proxy servers or special multicast Ethernet address		The solution does not require load balancers or dedicated proxy servers above the farm for imlementing load balancing. SafeKit is installed directly on the application servers in the farm. The load balancing is based on a standard virtual IP address/Ethernet MAC address and is working with physical servers or virtual machines on Windows and Linux without special network configuration This is not the case with network load balancers This is not the case with dedicated proxies on Linux This is not the case with a specific multicast Ethernet address on Windows
All clustering features		The solution includes all clustering features: virtual IP address, load balancing on client IP address or on sessions, monitoring of server/network/software failures, automatic application restart with a quick revovery time and a replication option with a mirror module This is not the case with other load balancing solutions. They are able to make load balancing but they do not include a full clustering solution with restart scripts and automatic application restart in case of failure. They do not offer a replication option The cluster configuration is very simple and made by means of application modules. There is no domain controller or active directory to configure on Windows. The solution works on Windows and Linux
Remote sites		If servers are connected to the same IP network through an extended LAN between remote sites, the virtual IP address of SafeKit is working with load balancing at level 2 If servers are connected to different IP networks between remote sites, the virtual IP address can be configured at the level of a load balancer. SafeKit offers a health check: the load balancer is configured with a URL managed by SafeKit which returns OK on the UP servers and NOT FOUND else. This solution is implemented for SafeKit in the Cloud but it can be also implemented with a load balancer on premise. Thus you can implement load balancing but also all the clustering features of SafeKit including an easy administration of the cluster through the SafeKit web console
Uniform high availability solution		SafeKit imlements a farm cluster with load balancing and failover. But it implements also a mirror cluster with replication and failover. Thus a N-tiers architecture can be made highly available and load balanced with the same solution on Windows and Linux (same installation, configuration, administration with the SafeKit console or with the command line interface). This is unique on the market This is not the case with an architecture mixing different technologies for load balancing, replication and failover

Real-time file replication and failover: click on the blue buttons
Mirror modules	Windows	Linux
Microsoft SQL Server		-
Oracle
MySQL
PostgreSQL
Firebird
Hyper-V		-
Milestone XProtect		-
Hanwha Wisenet SSM		-
New application
Amazon AWS mirror
Microsoft Azure mirror
Google GCP mirror
Cloud generic mirror