High Availability – System Failover for Small Businesses

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Overview
This post will demonstrate the abilities of the upcoming Synology High Availability via video demonstration at Interop New York City 2012, and display the network diagram for how this process is achieved.

Scenario
In this video, I am demonstrating the upcoming Synology High Availability (HA) package conducting a Live System Failover from one Synology XS-System to another (when two servers are joined in this manner, its typically referred to as a Cluster), while the HA Cluster is currently mounted with a VMware ESXi 5.1 Server with MPIO enabled.Ref: 1 This video was taken at Interop New York City 2012. High Availability provides system redundancy of the RackStation itself so that all services will failover to the secondary server if the first should suffer a failure (service failure, volume, or power).

Technical Details
For information of how this network was developed, please refer to this HA/MPIO Demo Network Diagram.Ref: 2

HA Cluster Details
As you can see, I’ve created a HA Cluster with two XS-Series RackStations. Data between these RackStations will be kept synced through the Heartbeat connection. Currently, an Active/Passive relationship is supported between two XS Models, where only one server (or Node) is Active at any given time. During a failure of the Active Node, the HA will execute a switch over to make the Passive Node to Active Node. The switch over process does take a few minutes to occur – and once completed – the five default services, SMB, AFP, FTP, NFS, and iSCSI, will be running on the Second Node. With Synology High Availability on the XS Series – it allows for great reduction in downtime should something happen to one of the RackStations, such as abnormal power, or multiple disk, or service failure.

MPIO Diagram
Also in the diagram is MPIO, in which case I’ve deployed two routers to achieve this. Yes, there are other ways MPIO can be achieved, such as using subnetting or multiple VLANs. However, both of these concepts, knowledge, and technology may not be readily accessible to a Small Business.

I admit that other experienced admins may dislike the idea of building a second network to achieve MPIO capability – however, it’s only a small portion of the data center side of a network and that the objective of this idea is to bring an affordable method of MPIO to the Small Business.

As in the diagram, the ESXi Server is connected to the Synology Clustered Addresses, not the individual IP address of the RackStation. From here, the concept of MPIO can be followed through, where data will be sent via Round Robin fashion to both Clustered IP Addresses. This not only provides greater throughput, but also provides Initiator Redundancy for the ESXi Server.

Timeout Values
During the failover process, there is a brief period where data from the Cluster is unavailable. For services that rely on file-level sharing clients will receive an error message that the target server is not responding or is not available. After the failover process has completed, the copy process simply needs to be restarted.

For services that use block-level storage the delay time during failover may cause the loss of data, as this is similar to a hard disk entering sector recovery mode.Ref: 3 To minimize the risk of data loss, the timeout value of the service should be set at a high value (such as 120 seconds). For VMware, the iSCSI Recovery Timeout can be set in the iSCSI Advanced Options.Ref: 4 For Windows, it is adjusted in the Registry Settings.Ref: 5

Conclusion
As seen in the video, Synology High Availability will become a powerful tool for Small Business, bringing an unforeseen level of Enterprise capability at a price which Small Businesses can afford and appreciate. Combined with the ability of conducting MPIO for iSCSI Targets, Synology continues its steady march as a software integrator, to bring Enterprise capabilities down for Small Business.

References
1. Synology Wiki – How to use iSCSI Targets on VMware ESXi with Multipath I/O
2. Synology HA/MPIO Demo Network Diagram, October 8 2012
3. Synology Blog – Which hard drive is best for my Synology DiskStation NAS?
4. VMware vSphere Blog: iSCSI Advanced Settings
5. MSDN: Registry Entries for SCSI Miniport Drivers

Further Reading
1. Synology Wiki – Synology High Availability