Instrument Manager™ (IM) Virtual Machine (VM) Snapshots for IT Administrators

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The following information is being provided as an extended reference to InterSystems External Freeze method. While some customers may choose to utilize this information, and we can support customers using Instrument Manager in a virtualized environment, we cannot support the virtualized environment itself or the use of 3rd party backup software. Data Innovations recommends using Mirroring provided by InterSystems™ Caché® to deliver a high availability and disaster recovery environment. Additional information about minimizing downtime with IM is available by contacting Data Innovations Customer Support.

Virtualization software allows you to create snapshots of running systems. You can create external InterSystems Caché database backups with virtual machine (VM) snapshots if you follow the correct procedures to avoid a service disruption or accidental High Availability failover.

Virtual machine snapshots are not backups by themselves. You must employ third-party backup software to work in conjunction with the snapshots. Third-party backup solutions use a snapshot process along with the backup application to manage the creation, and very importantly, the deletion of snapshots.

Snapshots apply to the entire virtual machine. The operating system, all applications, and the database engine are unaware that the backup is occurring.

If you take the snapshot without notifying the InterSystems Caché database engine, then the snapshot will be unusable for restoration. The database engine may have been attempting to modify the database files and/or journal files during the snapshot but couldn’t because the files were locked while the snapshot was being taken.

If you are using Instrument Manager’s High Availability feature, the Acting Primary may appear unresponsive to the other mirror members, and the mirror set may promote the other failover member. Once the snapshot process completes and the original Acting Primary becomes responsive again, there may be two systems believing that they are both the Primary.

To ensure the integrity of the snapshot, Caché provides methods to freeze and thaw writes to databases while the snapshot is being created. This ensures that the database files are not modified while the snapshot process takes place. User processes continue to perform memory updates without interruption.

Snapshots cannot include the virtual machine’s memory. The snapshot is typically of all file systems in use by the virtual machine. At a minimum, this includes all directories used by Caché in any way, including:

  • the installation directory
  • database directories
  • journal and alternate journal directories
  • WIJ (Write Image Journal) directory
  • any directory containing external files used by Caché

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