Why replicated clusters with DataKeeper are better than single copy SAN based clusters

If you have followed the history of clustering as closely as I have for the past 10 years as a Microsoft Cluster MVP, you will notice that Microsoft has been steadily been moving away from single copy clusters. It started with Windows Server 2003 with the elimination of a shared disk quorum and the introduction of majority node set quorums and the file share witness. The complaint with clusters based on shared disk quorums was that if the quorum became unavailable or corrupt, the entire cluster would fail. This was a major complaint and it primarily what gave clustering a bad name in the early days of clustering.

Once the shared disk quorum was eliminated, people were still left with their application data residing on the cluster which was also a problem as the SAN was still a single point of failure in a cluster, a performance impediment and a management headache. Microsoft has begun to address those concerns with the introduction of Exchange 2007 CCR and Exchange 2010 DAGs as well as SQL Server 2008 R2 Database Mirroring. Microsoft has eliminated Exchange 2010 single copy clusters entirely and SQL Server single copy clusters are only still around because they haven’t perfected SQL Server replication yet.

Hyper-V being the most recent cluster resource supported by Microsoft clustering does not yet have a native cluster integrated replication solution. This is where SIOS DataKeeper fits in. We first demonstrated our DataKeeper Hyper-V replication solution at the Microsoft Virtualization launch in September of 2008 and have been providing HA and DR solutions for Hyper-V since Hyper-V was first introduced. Our solution is logo certified for Windows Server 2008 R2 as well as Hyper-V.

DataKeeper fills the gap left by single copy clusters as show in the table below and subsequent paragraphs. The following customer story also highlights some of the reasons why people are adopting DataKeeper in lieu of SAN based solutions.


Eliminates single point of failure

A SAN is a single entity made up of redundant pieces. To have a truly redundant SAN you need redundant controllers, power supplies, CPU’s, switches, UPS, RAM and the clients connecting to it need to have redundant NICs or HBAs and multi-path solutions configured. Even once you have eliminated hardware as a single point of failure, the SAN is still controlled by firmware which itself is a single point of failure. And then because the SAN resides in a single location, any physical disasters (think water, fire, etc.) also represents a risk.

I/O Performance

Given same disk specs, disk installed locally will perform better than disks stored on a SAN accessed via iSCSI. Also, using local storage opens up the possibility of using even higher speed storage solutions such as flash based PCIe storage which outperforms SANs that costs hundreds of thousands of dollars at a fraction of the costs.


Not only do you have to facture in the initial investment, which the DataKeeper solution wins by a significant percentage, you have to factor in the ongoing expense involved with maintenance, power and cooling required for any enterprise class SAN.

Supports future expansion for Disaster Recovery

Should disaster recovery solutions become a requirement in the future the DataKeeper solution can easily accommodate adding an addition Hyper-V node in a remote location in a multisite cluster configuration for a robust disaster recovery solution that includes the best RTO and RPO available. The SAN solution would require the purchase of an additional SAN, replication software and might not even include cluster integration as there are only a few solutions that actually integrate with failover clustering as well as DataKeeper does.

Eliminates Planned Downtime

With SAN based cluster solutions, any maintenance on the SAN requires planned downtime. The DataKeeper solution allows for rolling upgrades, meaning planned downtime for hardware maintenance is eliminated.

Eases management

SAN administration usually involves a SAN administrator who is familiar with the features and functionality of a SAN. The DataKeeper solution on the other hand is a simple software solution that is managed by the Windows Server administrator and features complete integration with Windows Server Failover Clustering, meaning the management is controlled through failover cluster, a tool which should be familiar with most Windows Administrators.


In summary, DataKeeper is able to provide a much more resilient cluster solution at a fraction of the cost of SAN based solutions.

Why replicated clusters with DataKeeper are better than single copy SAN based clusters

SQL Server 2012 AlwaysOn Multisite Failover Cluster Instance White Paper

Here is an excellent white paper on SQL Server Multisite Clusters, however they forget to mention that you can also do this with host based replication. Instead, they assume you have “two EMC Symmetrix VMAX enterprise storage arrays, one at each site. The arrays were both configured with two VMAX storage engines and 240 disk drives”. If you have a million plus dollars in your budget for storage, go ahead and knock yourself out. If not, you may want to look into some Fusion-io PCIe Flash storage and host based replication with DataKeeper cluster edition, faster than a SAN at a fraction of the cost with all the availability. Check out how Polaris Industries did just this http://www.fusionio.com/blog/polaris-sios/



SQL Server 2012 AlwaysOn Multisite Failover Cluster Instance White Paper