Restoring a DFS (Distributed File System) partner a month after a significant hardware failure can lead to complex and potentially problematic scenarios. Here's a breakdown of the key concerns:

Data Inconsistency:

• Replication Conflicts:
  
  
  • The restored server will have a snapshot of the data from a month ago. Meanwhile, the other DFS partners have continued to replicate changes. This will create significant conflicts.
  
  
  • DFS Replication (DFSR) will attempt to reconcile these differences, but this process can be lengthy and resource-intensive.
  
  
  • There's a high risk of data loss or corruption if conflicts are not handled correctly.
  
  
  
  


• Data Loss:
  
  
  • Any changes made to the data within that month on the other DFS partners will be at risk of being overwritten by the older data from the restored server.
  
  
  • Conversely, any data that was on the restored server and has been deleted or modified on the other servers will be re-introduced.
  
  
  
  

Potential Issues:

• Replication Backlog:
  
  
  • The restored server will need to catch up with a month's worth of changes. This can create a massive replication backlog, potentially overloading network resources.
  
  
  
  


• Active Directory Issues:
  
  
  • DFS relies on Active Directory for its configuration. If Active Directory has changed significantly in the past month, the restored server's configuration may be out of sync, leading to errors.
  
  
  
  


• DFSR Database Corruption:
  
  
  • The DFSR database on the restored server may be incompatible with the current state of the other DFS partners, leading to corruption and replication failures.
  
  
  
  


• Namespace Inconsistencies:
  
  
  • If the DFS Namespace has been modified in the past month, the restored server may present an outdated view of the namespace, confusing users.
  
  
  
  

Recommendations:

• Avoid a Direct Restore:
  
  
  • If possible, avoid restoring the server to its exact state from a month ago.
  
  
  
  


• Prioritize Data Recovery:
  
  
  • Focus on recovering critical data from the backup, rather than restoring the entire server.
  
  
  • Carefully evaluate which data is most important and prioritize its recovery.
  
  
  
  


• Careful Replication Management:
  
  
  • If a restore is unavoidable, closely monitor the DFSR replication process.
  
  
  • Use DFSR diagnostic tools to identify and resolve conflicts.
  
  
  • It may be necessary to do an initial non replicating data copy, followed by a new DFS replication setup.
  
  
  
  


• Validate Data Integrity:
  
  
  • Thoroughly validate the integrity of the data after the restore and replication process.
  
  
  
  


• Consider a New DFS Partner:
  
  
  • In many cases, it may be cleaner to build a new DFS partner and then copy the needed data to it. This would remove the risk of the old DFS Databases causing issues.
  
  
  
  


• Backup Verification:
  
  
  • This situation highlights the importance of regular backup verification. It is very important to make sure that backups are working, and restorable.
  
  
  
  

In summary: Restoring a DFS partner from a month-old backup after a major failure is highly risky. It can lead to data loss, replication issues, and namespace inconsistencies. Careful planning and execution are essential to minimize these risks.

The reason why "new" data doesn't simply and cleanly overwrite the "old" data in a DFS Replication (DFSR) scenario, especially after a significant time gap like a month, comes down to how DFSR handles changes and conflicts. Here's a breakdown:

DFSR Conflict Resolution:

• "Last Writer Wins":
  
  
  • DFSR primarily uses a "last writer wins" conflict resolution method. This means that when two versions of a file exist on different servers, the version with the most recent modification timestamp is typically considered the winner.
  
  
  • However, this is not always a simple process. DFSR has to evaluate many factors.
  
  
  
  


• Version Vectors and Change Tracking:
  
  
  • DFSR maintains metadata about file versions and changes. This metadata helps it determine which version is more recent and which changes need to be replicated.
  
  
  • When a server is restored from a month-old backup, its version vectors are out of sync with the rest of the replication group. This creates a situation where DFSR has to reconcile a large number of conflicting changes.
  
  
  
  


• Conflict and Deleted Folder:
  
  
  • When DFSR detects a conflict, it doesn't always simply overwrite one version with the other. Instead, it often moves the losing version to a "Conflict and Deleted" folder. This preserves the conflicting data, allowing administrators to review and recover it if necessary.
  
  
  • This is a key point. DFS-R is designed to preserve data, not simply delete it.
  
  
  
  


• Replication Backlog and Staging:
  
  
  • The restored server has to process a month's worth of changes. This creates a massive replication backlog. DFSR uses staging folders to temporarily store files during the replication process.
  
  
  • The process of working through this back log takes time, and can cause many conflicts, as the restored server attempts to bring its self up to date.
  
  
  
  

Why It's Complicated:

• Not Just File Dates:
  
  
  • DFSR doesn't rely solely on file modification dates. It also tracks other metadata and changes to ensure data consistency.
  
  
  
  


• Active Directory Integration:
  
  
  • DFSR relies on Active Directory for its configuration. Changes to Active Directory can affect the replication process and create further complications.
  
  
  
  


• Database Integrity:
  
  
  • DFSR uses a database to track replication changes. An old database can cause inconsistencies and errors.
  
  
  
  

In essence:

• DFSR is designed to be a robust replication system that prioritizes data integrity. Therefore, it has built-in mechanisms to handle conflicts and prevent data loss.


• These mechanisms, while essential, can make the process of reconciling data after a significant time gap complex and time-consuming.

Therefore, rather than a simple overwrite, DFS-R will attempt a complex reconciliation, that has a high potential for errors, and conflict.

While using robocopy might seem like a quicker and simpler solution than letting DFS Replication (DFSR) handle the reconciliation, it's generally not recommended as a primary strategy after a significant hardware failure and month-long data discrepancy. Here's why:

Problems with robocopy Overwrites:

• Loss of DFSR Metadata:
  
  
  • robocopy is a file-copying tool, not a replication tool. It doesn't preserve DFSR metadata, such as version vectors and change tracking information. This means that after a robocopy overwrite, DFSR will lose its ability to track changes effectively, leading to potential inconsistencies and replication problems.
  
  
  
  


• Conflict Resolution Issues:
  
  
  • robocopy will overwrite files based on its specified parameters (e.g., last modified date). It doesn't have the sophisticated conflict resolution mechanisms of DFSR. This can lead to unintended data loss or overwrites.
  
  
  
  


• Active Directory and DFSR Database Inconsistencies:
  
  
  • robocopy only copies files. It doesn't address potential inconsistencies in Active Directory or the DFSR database. This can lead to further replication problems.
  
  
  
  


• Permissions Issues:
  
  
  • Robocopy can copy permissions, but it is another area where errors can occur. If the source and destination servers have different security principals, or any differences in the active directory, permissions issues will occur.
  
  
  
  


• Potential for Data Corruption:
  
  
  • If the source data is corrupted, robocopy will simply copy the corrupted files to the destination.
  
  
  
  


• No Versioning:
  
  
  • Robocopy will overwrite the files. There will be no record of the old files, unless you specifically set up robocopy to archive the old files.
  
  
  
  

When robocopy Might Be Useful (with Caution):

• Initial Data Seeding:
  
  
  • In some cases, robocopy can be used to perform an initial data seeding before enabling DFSR. This can speed up the initial replication process. However, this should be done carefully and only when you're sure the source data is clean and consistent.
  
  
  
  


• Data Recovery (Specific Files):
  
  
  • If you need to recover specific files or folders from a backup, robocopy can be a useful tool. However, this should be done selectively and with careful consideration of potential conflicts.
  
  
  
  


• Post conflict resolution:
  
  
  • After the DFS-R conflict resolution has completed, and the administrator has reviewed the conflict and deleted folder, then robocopy could be used to copy data from the conflict and deleted folder, into the main file structure.
  
  
  
  

Better Alternatives:

• Careful DFSR Reconciliation:
  
  
  • The best approach is usually to allow DFSR to handle the reconciliation process, even if it takes time. Monitor the process closely and use DFSR diagnostic tools to identify and resolve conflicts.
  
  
  
  


• New DFS Partner:
  
  
  • If possible, build a new DFS partner and copy the necessary data. This avoids the complexities of reconciling an old server.
  
  
  
  


• Backup Software Restore:
  
  
  • Use the backup software's restore functions to restore the data. Most backup software is designed to restore files and folders while maintaining integrity.
  
  
  
  

In summary: While robocopy is a powerful tool, it's not a suitable replacement for DFSR's replication mechanisms. Using it to overwrite shares after a major failure can lead to data loss, replication problems, and other issues. Prioritize DFSR reconciliation or consider building a new DFS partner.