Virtual Disk Bad Blocks

Not only do physical drives have bad blocks, but virtual drives can have bad blocks too – and that restricts the VM from accessing the files on the virtual drive. But most times, if your VM is showing that the virtual drive has bad blocks, it could be a false positive. However, whether the error report you’re seeing is true or a false positive, this article explains how to deal with virtual disk bad block issues.

Identifying Bad Blocks in Virtual Disks

Symptoms and Impact

Symptoms of Bad Blocks in Virtual Disks:

• Slow Performance: One of the first signs of bad blocks in a virtual disk is a noticeable decrease in system performance, especially during read/write operations.
• Frequent Errors: Applications or the operating system might generate frequent error messages related to file access, such as “File cannot be read,” “Disk I/O error,” or “File is corrupt.”
• System Crashes or Freezes: The virtual machine (VM) may crash unexpectedly or freeze during certain operations, especially when trying to access files located on bad blocks.
• Data Corruption: Files stored on bad blocks may become corrupted, leading to unexpected behavior in applications or loss of critical data.
• Inability to Boot: If the bad blocks are located in system-critical areas of the virtual disk, such as the boot sector, the VM may fail to boot entirely.
• Disk Space Anomalies: You might notice discrepancies in reported disk space, with some areas of the virtual disk appearing full when they shouldn't be.

Impact of Bad Blocks on Virtual Disks:

• Data Loss: The most significant impact is the potential loss of data stored on the bad blocks. This can include critical system files or important user data.
• Reduced Performance: Bad blocks can cause significant performance degradation as the system repeatedly attempts to read or write to these problematic areas.
• Increased System Instability: Persistent bad blocks can lead to frequent crashes, making the VM unreliable for production or critical tasks.
• Corrupted Backups: If backups are made from a virtual disk with bad blocks, the backups themselves may also be corrupted, complicating recovery efforts.

Diagnostic Tools

CHKDSK (Windows):

• Purpose: CHKDSK is a built-in Windows utility that can be used to scan and repair bad sectors on virtual disks used by Windows-based VMs.
• Usage: Running chkdsk /f /r within the VM can help detect and fix bad blocks. The /f flag fixes errors, while the /r flag locates bad sectors and recovers readable information.

fsck (Linux):

• Purpose: The fsck (File System Consistency Check) utility is the Linux equivalent of CHKDSK and can be used to identify and fix bad blocks on Linux-based virtual disks.
• Usage: Running fsck -c checks for bad blocks. The -y option can be used to automatically fix any errors found.

SMART Monitoring Tools:

• Purpose: Although traditionally used for physical disks, Self-Monitoring, Analysis, and Reporting Technology (SMART) can sometimes be applied to virtual disks, depending on the underlying storage infrastructure.
• Usage: Tools like smartctl can be used to monitor the health of the physical disk hosting the virtual disk, indirectly providing insights into potential bad block issues.

Hypervisor Tools:

• Purpose: Many hypervisors, such as VMware vSphere or Microsoft Hyper-V, provide built-in tools for monitoring and managing virtual disks.
• Usage: These tools may include disk integrity checks, performance monitoring, and error reporting that can help identify bad blocks.

Third-Party Disk Utilities:

• Purpose: Specialized third-party tools, like SpinRite or HDD Regenerator, can be used to perform deeper scans and repairs of bad blocks, even on virtual disks.
• Usage: These tools may require the virtual disk to be mounted or accessed through the host system for thorough diagnostics.

Virtual Disk Management Software:

• Purpose: Software specifically designed for managing virtual disks, such as Veeam or Acronis, often includes features for checking disk integrity and identifying bad blocks.
• Usage: These tools can provide automated scans and detailed reports on the health of virtual disks.

Prevention and Best Practices

Regular Maintenance

Routine Disk Checks:

• Scheduled Disk Scans: Regularly run disk checking utilities like CHKDSK (Windows) or fsck (Linux) to detect and fix minor issues before they escalate into bad blocks.
• Hypervisor Maintenance Tools: Utilize the maintenance and diagnostic tools provided by your hypervisor (e.g., VMware vSphere, Microsoft Hyper-V) to monitor virtual disk health and identify potential problems early.

Disk Defragmentation:

• Windows Defragmentation: For virtual machines running on Windows, regular disk defragmentation can help organize data and reduce the likelihood of bad blocks due to file system fragmentation.
• Linux File System Optimization: In Linux environments, consider using file systems like EXT4, which have built-in optimization features, and run maintenance tools like e4defrag when necessary.

Monitoring Disk Health:

• SMART Monitoring: If the virtual disk resides on a physical disk that supports SMART (Self-Monitoring, Analysis, and Reporting Technology), monitor the physical disk’s health to catch issues that might lead to bad blocks in the virtual environment.
• Hypervisor Alerts: Set up alerts within your hypervisor to notify you of any unusual disk activity or errors that could indicate developing bad blocks.

Keep Software Updated:

• Operating System Updates: Ensure that the operating systems running on your virtual machines are up-to-date with the latest patches and updates, as these often include fixes for file system and disk-related issues.
• Hypervisor and Virtualization Tools: Regularly update your hypervisor software and any associated virtualization tools to benefit from the latest performance improvements and bug fixes.

Storage Optimization:

• Disk Space Management: Avoid overfilling virtual disks and allocate adequate space for growth. Running out of disk space can lead to corruption and bad blocks.
• Storage Tiering: If possible, use storage tiering strategies where critical virtual disks are stored on higher-performance, more reliable storage media.

Backup Strategies

Regular Backups:

• Automated Backup Schedules: Implement automated backup schedules to regularly back up virtual disks, ensuring that you always have a recent restore point in case of failure.
• Full and Incremental Backups: Use a combination of full and incremental backups to optimize storage space while ensuring comprehensive coverage. Full backups capture the entire disk, while incremental backups only capture changes.

Offsite and Cloud Backups:

• Offsite Backup Storage: Store backups in a secure offsite location to protect against data loss due to physical damage or disasters at the primary site.
• Cloud-Based Backup Solutions: Consider using cloud-based backup services for virtual disks, which offer scalability, redundancy, and quick recovery options.

Snapshot Management:

• Regular Snapshots: Create regular snapshots of virtual machines to provide quick recovery points in case of disk issues. Snapshots can be particularly useful for short-term protection against bad blocks.
• Snapshot Retention Policy: Implement a retention policy to manage the number of snapshots stored, ensuring that old snapshots are deleted to free up space and reduce potential performance issues.

Testing Backups:

• Regular Backup Verification: Regularly test your backups by restoring them to a separate environment. This ensures that the backups are reliable and can be used for recovery if needed.
• Automated Backup Validation: Use tools that automatically verify the integrity of backups post-creation to ensure that no corrupted data is stored.

Redundant Storage Solutions:

• RAID Configurations: Implement RAID (Redundant Array of Independent Disks) configurations for your virtual disk storage. RAID provides redundancy, improving data availability and reducing the risk of data loss due to bad blocks.
• Hybrid Storage Approaches: Combine SSDs and HDDs in your storage infrastructure to balance performance and reliability, with critical data stored on SSDs and backups or less critical data on HDDs.

Conclusion

This case study demonstrates the significant impact that bad blocks can have on a VMware environment, particularly when critical business operations are at stake. Through careful diagnosis, strategic use of specialized recovery tools, and a methodical approach to problem-solving, the company was able to successfully recover their virtual machines and restore full functionality to their VMware environment.

The key takeaway from this experience is the importance of proactive monitoring, regular maintenance, and having the right tools at your disposal when facing disk-related issues. DiskInternals VMFS Recovery proved to be an invaluable resource in resolving the bad block issues, allowing the IT team to recover lost data and mitigate the risk of further data corruption.

By implementing stronger backup strategies and ongoing maintenance practices, the company is now better equipped to handle potential disk issues in the future, ensuring business continuity and data integrity in their virtualized infrastructure. This case underscores the critical need for robust data recovery solutions and the value they bring in maintaining the resilience of enterprise IT systems.