Byte-level file replication vs block-level disk replication

Byte-level file replication vs block-level disk replication

Byte-level file replication vs block-level disk replication is an important point to consider when choosing a cluster solution

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

Byte-level file replication

Cluster with block-level disk replication

Block-level disk replication

ProductSafeKit on Windows LinuxDisks replication products
Application data organization0 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.
All data to recover must be localized in a special replicated disk.
Data in the system disk cannot be replicated.
Data replicationSynchronous byte-level file replication
Replication at the file operation level.
Replicate file modification operations generated by application activity : no meta data are replicated. Only data modified in files are replicated, not entired 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 deploymentNo - install a software on 2 serversYes - require specific IT skills to configure OS and replicated disk
FailoverJust restart the application on the secondary serverRemount the file system on the replicated disk.
Pass the recovery procedure on the file system.
And then restart the application
FailbackAutomatic 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 quorumApplication executed on a single server after a network isolation and repair (split brain).
Coherency of data after a split brain.
No need of a third machine or a quorum disk or a special hearbeat line for split brain. Cluster quorum video : no quorum server and no quorum disk		All products are not at the same level of features
Suited forSoftware editors which want to add a simple high availabity to their applicationEnterprise with IT skills

Byte-level file replication example with a Microsoft SQL Server 2012 cluster

More information on a byte-level file replication cluster on Windows and Linux here.

Key differentiators of file replication and failover with the Evidian SafeKit mirror cluster

Evidian SafeKit mirror cluster with real-time file replication and failover

Synchronous replication Synchronous replication

Like  The real-time replication is synchronous with no data loss on failure

Dislike  This is not the case with asynchronous replication

After a server failure, fully automated failback procedure Automatic failback

Like  After a server failure and a failover, the replication failback procedure is fully automatic on the failed server and without stopping the application on the only remaining server

Dislike  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

All clustering features All clustering features

Like  The solution includes all clustering features: server failure monitoring, network failure monitoring, software failure monitoring, automatic application restart with a quick recovery time, a virtual IP address switched in case of failure to automatically reroute clients. A clustering configuration is simply made by means of a high availability application module. There is no domain controller or active directory to configure on Windows. The solution works on Windows and Linux

Dislike  This is not the case with replication-only solutions like replication at the database level

Dislike  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 an unknown recovery time

Replication of any type of data

Like  The replication is working for databases but also for any files which shall be replicated

Dislike  This not the case for replication at the database level

File replication vs disk replication File replication vs disk replication

Like  The replication is based on file directories that can be located anywhere (even in the system disk)

Disike  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 File replication vs shared disk

Like  The servers can be put in two remote sites

Dislike  This is not the case with shared disk solutions

Remote sites Remote sites

Like  With remote sites, the solution works with only 2 servers and for the quorum (network isolation), a simple split brain checker is offered

Like  This is not the case for most clustering solutions where a 3rd server is required for the quorum

Like  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

Like  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

Like  Virtual IP address is not offered by replication-only solution

Uniform high availability solution Uniform high availability solution

Like  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 me 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

Dislike  This is not the case with an architecture mixing different technologies for load balancing, replication and failover

FAQ on Evidian SafeKit (byte-level file replication)

3 demonstrations [+]

Application high availability modules [+]

Cloud solutions [+]

Customers [+]

Best use cases [+]

Distinctive advantages [+]

More on the mirror cluster [+]

More on the farm cluster [+]

SafeKit Webinar [+]

Pricing - Free trial [+]