Windows Server Performance Hit: Full Garbage Collection Blamed

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This document addresses performance degradation issues encountered in Windows Server environments, specifically those arising from the intensive operations of full garbage collection during data deduplication processes. These performance bottlenecks can significantly impact system responsiveness and overall efficiency, particularly in environments dealing with substantial data volumes and frequent data modifications. Understanding the root causes and implementing effective workarounds is crucial for maintaining optimal server performance.

Symptoms¶

Full garbage collection, while designed to reclaim more disk space compared to regular garbage collection, introduces a higher degree of churn on the storage volume. This increased churn stems from the process of compacting every chunk container if any unreferenced data chunks are detected within it. This rewriting of chunk containers, even when only a small portion contains garbage, leads to a cascade of effects that manifest as various performance problems.

The excessive churn generated by full garbage collection can trigger a series of adverse symptoms, including:

• Deletion of Volume Shadow Copy Service (VSS) shadow copies: The intense I/O activity associated with full garbage collection can consume significant storage space within the shadow copy storage area. If this area becomes exhausted, VSS may automatically delete older shadow copies to free up space. This loss of shadow copies can compromise data recovery and point-in-time restore capabilities.

• Heavy I/O load on the system: Full garbage collection operations are inherently I/O-intensive. When combined with already demanding deduplication workloads or other I/O-intensive applications running on the server, this can lead to excessive disk I/O. Elevated I/O can saturate disk resources, resulting in slow response times for applications and overall system sluggishness. This is particularly pronounced on systems with traditional spinning disks rather than faster solid-state drives (SSDs).

• Increased volume workloads for incremental solutions: Solutions that rely on tracking file changes, such as incremental backups and file replication services, can experience inflated workloads due to the file churn caused by full garbage collection. Even if user-level files are not modified, the underlying rewriting of chunk containers is perceived as file changes by these solutions. This leads to larger backup sizes, longer replication times, and increased network bandwidth consumption. The apparent ‘churn’ at the file system level is a consequence of the deduplication process’s internal operations.

Cause¶

The observed performance issues related to full garbage collection are typically triggered by specific operational patterns and system configurations. Understanding these underlying causes is essential for implementing targeted and effective workarounds.

The problematic behavior is often observed in the following scenarios:

• Workloads with frequent file deletions or in-place writes: Environments characterized by a high volume of file deletions or modifications directly contribute to the accumulation of unreferenced data chunks. When files are deleted or overwritten, the chunks they previously occupied become unreferenced and eligible for garbage collection. Full garbage collection, triggered in such environments, will aggressively attempt to reclaim this space, leading to the aforementioned churn. The problem is exacerbated when deletes result in many chunk containers containing a mix of old and new chunks, as these are prime candidates for compaction during full garbage collection.

• Systems with limited physical free space: When a system is running with minimal free disk space, the NTFS file system’s space allocation behavior can further amplify the issues caused by full garbage collection. NTFS prioritizes using free space that does not impact shadow copy storage consumption initially. However, when available free space is scarce, NTFS is forced to allocate space for new files in areas that trigger “copy-on-write” behavior. This means that even small file modifications can result in significant disk activity as NTFS must copy existing data to new locations before writing the changes. Subsequently, when the shadow copy storage area reaches its capacity, VSS will proactively delete shadow copies to maintain system stability, directly impacting data protection capabilities. This interplay between low free space, NTFS behavior, and VSS management creates a perfect storm when combined with aggressive full garbage collection.

Workaround¶

To mitigate the performance problems associated with full garbage collection during deduplication, several effective workarounds can be implemented. These strategies focus on either isolating the impact of garbage collection or modifying its behavior to reduce its intensity.

Here are recommended methods to alleviate these issues:

1. Configure VSS to Utilize a Separate Volume for Shadow Storage:

   

   One highly effective approach is to dedicate a separate storage volume, potentially even a dedicated physical disk, specifically for the Volume Shadow Copy Service (VSS) diff area, also known as the shadow storage area. By isolating the shadow storage onto a distinct volume, the I/O churn generated by full garbage collection on the primary data volume is less likely to interfere with VSS operations. This separation prevents the exhaustion of shadow storage space caused by garbage collection’s I/O load, thereby mitigating the risk of automatic shadow copy deletion.

   This configuration can be achieved using the Vssadmin.exe command-line tool or through graphical interfaces within Windows Server’s management tools. For example, using vssadmin add shadowstorage command. Detailed instructions on configuring VSS shadow storage can be found in Microsoft’s official documentation.

   Note: Beyond resolving the immediate issue of shadow copy deletion during full garbage collection, deploying a dedicated volume for the VSS diff area offers broader performance advantages. Separating VSS I/O from primary data volume I/O can improve overall system responsiveness and reduce contention for disk resources, particularly in environments with heavy I/O workloads.

2. Restrict Deduplication to Regular Garbage Collection Mode:

   

   Another effective workaround involves configuring data deduplication to execute garbage collection exclusively in “regular” mode, effectively preventing the execution of full garbage collection jobs. By default, Windows Server schedules garbage collection jobs to run on a weekly basis. Furthermore, the default configuration dictates that every fourth garbage collection job is performed as a full garbage collection, occurring on a monthly cadence. This monthly full garbage collection is the primary source of the performance issues discussed.

   By disabling full garbage collection and relying solely on regular garbage collection, the intensity of disk churn is significantly reduced. Regular garbage collection is less aggressive and compacts chunk containers less frequently than full GC, leading to lower I/O overhead and reduced risk of triggering the described symptoms.

   Note: While preventing full garbage collection mitigates the performance problems, it’s important to understand the trade-offs. Regular garbage collection is not as thorough as full garbage collection in reclaiming unreferenced data. Over time, some unreferenced deduplication chunks may not be reclaimed if full garbage collection is never performed. However, regular garbage collection is still highly effective and is estimated to reclaim over 95 percent of unreferenced data. For many workloads, this slightly less aggressive approach is an acceptable trade-off for improved system stability and performance.

   Full garbage collection can still be initiated on demand if needed. Administrators retain the flexibility to manually trigger a full garbage collection cycle when system resources are less constrained or during scheduled maintenance windows. This can be done using the following Windows PowerShell command:

   Start-DedupJob <volume> -Type GarbageCollection -Full
   

   Replace <volume> with the drive letter or volume name where deduplication is enabled. This command provides granular control over when full garbage collection is executed, allowing administrators to balance space reclamation with performance considerations.

   To permanently prevent scheduled full garbage collection from running automatically, you can configure a specific registry key. For standalone Windows Server systems, modify the following registry value:

   HKLM\System\CurrentControlSet\Services\ddpsvc\Settings
   Value Name: DeepGCInterval
   Type: REG_DWORD
   Data: 0xffffffff (Decimal: 4294967295)
   

   For clustered Windows Server environments, the relevant registry key is:

   HKLM\CLUSTER\Dedup\
   Value Name: DeepGCInterval
   Type: REG_DWORD
   Data: 0xffffffff (Decimal: 4294967295)
   

   Setting the DeepGCInterval value to 0xffffffff effectively disables scheduled full garbage collection by setting the interval to a very large number, essentially preventing it from ever occurring automatically based on the default schedule.

   By implementing either or both of these workarounds, administrators can effectively address the performance issues associated with full garbage collection in Windows Server deduplication environments. Choosing the most appropriate workaround depends on the specific environment and the relative importance of immediate performance versus long-term space reclamation thoroughness. In many cases, disabling scheduled full garbage collection while retaining the option to run it manually as needed offers a balanced and practical solution.

We encourage you to share your experiences and questions in the comments below. Have you encountered similar performance issues with garbage collection in your Windows Server environment? What workarounds have you found effective? Your insights can be valuable to other readers facing similar challenges.