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


Cluster with byte-level file replication

Byte-level file replication

Cluster with block-level disk replication

Block-level disk replication

ProductSafeKit on Windows Linux AIXDisks 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, Linux and AIX here.

More comparisons to understand how to achieve simplicity in high availability


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