High-Integrity Data Protection
SNAP HA™ offers two methods of data protection, based on the storage type selected at creation, Standard or Shared Pools:
Several measures have been taken to ensure the highest possible data integrity of your highly available block storage system. An independent "witness" HA controller function ensures there is never a condition that can result in what is known as "split-brain", where a controller with outdated data is accidentally brought online. SNAP HA™ prevents split-brain using a number of industry-standard best practices, including use of a 3rd party witness HA control function that tracks which node contains the latest data. On AWS, shared data stored in highly-redundant S3 storage is used. On Azure, blob storage is leveraged. On VMware, a separate HA Controller VM is used.
Another HA feature is "fencing". In the event of a node failure or takeover, the downed controller is shut down and fenced off, preventing it from participating in the cluster until any potential issues can be analyzed and corrected, at which point the controller can be admitted back into the cluster.
Finally, data synchronization integrity checks prevent accidental failover or manual takeover by a controller which contains data which is out of date.
The combination of high-integrity features built into SNAP HA™ ensures data is always protected and safe, even in the face of unexpected types of failures or user error.
Even with these strong measures in place, limited data loss (approximately 5 seconds worth) can occur at the moment of failure if default settings for SoftNAS' implementation are used. This risk is present to a varying degree in any high availability solution relying on the real-time transfer of active data between two nodes. SoftNAS' default settings are in place to provide a balance between performance and data integrity concerns. Measures can be taken when creating pools and volumes for high availability to limit or eliminate this potential loss. Sync mode settings can be used to further enforce data integrity, but with a hit to performance. SoftNAS strongly recommends the creation of a write log, or ZIL to cache high bursts of write activity, and further protect data integrity, as well as boosting performance.