SQL Server Data Collector: Diagnose Upload Failures and Ensure Data Integrity

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Microsoft SQL Server Data Collector is a powerful tool designed to gather information about server performance and activity. This data is typically cached locally before being uploaded to a central Management Data Warehouse (MDW) database for reporting and analysis. However, a common issue encountered by administrators is the failure of collection sets to upload new data, often indicated by specific errors in the collection set logs. One significant cause for these upload failures is corruption within the Data Collector’s local cache files.

When a Data Collector collection set is configured, it captures performance and configuration data according to its definition. This captured data is temporarily stored in raw data files within a designated cache directory. The upload process then reads these cached files, typically using SQL Server Integration Services (SSIS), and transfers the data to the tables in the Management Data Warehouse. If the integrity of these cached files is compromised, the SSIS process responsible for uploading the data can fail, leading to the symptoms described. Understanding the role of these cache files and how they can become corrupted is crucial for effective troubleshooting and resolution.

Understanding Data Collector Cache File Corruption¶

The Data Collector cache serves as a buffer, holding collected data before it’s moved to the more permanent storage of the Management Data Warehouse. This caching mechanism helps to ensure that data collection can proceed even if the network connection to the MDW is temporarily unavailable or the MDW database is offline. The data is stored in a specialized raw file format that is highly optimized for writing data streams efficiently. During the upload phase, an SSIS package is dynamically generated and executed to read these raw files and load the data into the MDW tables.

The errors observed when cache files are corrupted are often reported by the SSIS components attempting to read the raw data files. These errors typically point to issues with the file structure, metadata, or data types, indicating that the file does not conform to the expected SSIS raw file format. Because the upload process relies entirely on successfully reading these cache files, any corruption immediately halts the transfer of data to the MDW, resulting in a backlog of un-uploaded data in the cache directory. Identifying and addressing the corrupted files is therefore the primary step in resolving the upload failure.

Symptoms of Cache File Corruption¶

When Data Collector cache files are corrupted, the most direct symptom is the cessation of data uploads to the Management Data Warehouse. Administrators will typically discover this issue by checking the Data Collector collection set logs within SQL Server Management Studio (SSMS). These logs provide detailed information about the execution of collection and upload jobs. Upon reviewing the logs for the affected collection set, one or more error messages similar to the following will be present:

• Error Message 1:
  > The file had bad version and flags information. The file is damaged or not a SSIS-produced raw data file.
  > component “Raw File Destination” (57) failed the pre-execute phase and returned error code 0xC0202061

  This specific error indicates that the file header, which contains crucial information about the file format, version, and internal flags used by SSIS, is incorrect or unreadable. SSIS expects a specific structure at the beginning of a raw data file; a mismatch suggests the file was not properly written or has been altered externally in a way that corrupts this header information. The “pre-execute phase” failure means the SSIS component couldn’t even prepare to read the file due to this fundamental header issue.

• Error Message 2:
  > Encountered bad metadata in file header. The file is damaged or not a SSIS-produced raw data file.
  > component “Raw File Destination” (57) failed the pre-execute phase and returned error code 0xC020205E

  Similar to the first error, this message also points to problems with the file header. “Metadata” in this context refers to information embedded in the file that describes the data structure, such as column definitions, data types, and row counts. Corrupted metadata means SSIS cannot correctly interpret the layout of the data within the file, making it impossible to read and process. This also prevents the upload process from starting correctly.

• Error Message 3:
  > Unexpected end-of-file encountered while reading X bytes from file “Y”. The file ended prematurely because of an invalid file format.
  > component “Raw File Destination” (57) failed the pre-execute phase and returned error code 0xC0202069

  This error suggests that the file abruptly terminates before SSIS expects it to. When reading a raw file, SSIS relies on internal markers or file size information to determine the end of the data stream. An “unexpected end-of-file” condition implies that the file was truncated or incomplete, possibly due to an interruption during the writing process. This could happen if the collection process was terminated unexpectedly or if there was an issue with the storage medium while the file was being written.

• Error Message 4:
  > The adapter encountered an unrecognized data type of X. This could be caused by a damaged input file (source) or by an invalid buffer type (destination).
  > component “Raw File Destination” (57) failed the pre-execute phase and returned error code 0xC020206B

  Raw files store data efficiently using specific internal data type identifiers. This error occurs when SSIS reads a value that does not correspond to a recognized internal data type identifier. This typically indicates that the data within the file has been corrupted, leading to binary patterns that SSIS misinterprets as invalid type information. Such corruption could affect individual data points within the file, rendering the entire file unreadable by the SSIS component.

• Error Message 5:
  > String too long. The adapter read a string that was X bytes long, and expected a string no longer than Y bytes, at offset Z. This could indicate a damaged input file. The file shows a string length that is too large for the buffer column.
  > component “Raw File Destination” (57) failed the pre-execute phase and returned error code 0xC020206C

  This error is specific to string or variable-length data types stored in the raw file. The file format includes length prefixes for strings. If this length prefix is corrupted, it might indicate an excessively large length, causing SSIS to attempt to read beyond the bounds of the file or a predefined buffer size. This is a clear indicator of data corruption within the file contents, specifically affecting how string data is encoded and interpreted by SSIS.

Each of these error messages, while slightly different, fundamentally points to a problem with the structure or content of the Data Collector’s raw cache files. They all occur during the “pre-execute phase” of the SSIS task responsible for reading the raw file, highlighting that the issue prevents the data extraction from even beginning properly.

Causes of Cache File Corruption¶

Cache file corruption in the SQL Server Data Collector can stem from various issues that interrupt or interfere with the process of writing data to disk. Understanding these causes can help in preventing future occurrences once the immediate problem is resolved. While a definitive root cause can sometimes be hard to pinpoint after the fact, common culprits include:

1. Data Collector Encountered an Exception: Errors or unexpected events occurring within the Data Collector process itself while it is actively writing data to the cache file can lead to incomplete or improperly formatted file writes. An uncaught exception, a crash, or a sudden termination of the collection process can leave a cache file in an inconsistent state. Such exceptions might be triggered by issues within the SQL Server engine, the operating system, or the specific queries being executed by the collection items.

2. The Disk Runs Out of Free Space: This is a very common cause of file corruption. If the disk volume where the cache directory is located runs out of space while Data Collector is writing data, the operating system will fail to allocate the necessary space to complete the write operation. This results in partial or corrupted files. Future attempts to write to the file or read it will likely encounter errors due to the incomplete write. Monitoring disk space is critical for any system that relies on writing data to disk, including Data Collector.

3. A Firmware or a Driver Problem Occurs: Issues with the storage subsystem, including faulty disk drives, malfunctioning storage controllers, or outdated/buggy firmware or drivers for these components, can cause data to be written incorrectly or to become corrupted on disk. These low-level issues can manifest as file system corruption or silent data corruption, affecting any files being actively written to or read from the affected storage. Ensuring that storage hardware and its associated drivers/firmware are healthy and up-to-date is essential for data integrity.

Other potential, though less frequent, causes could include sudden power loss, hardware failures (like failing RAM that causes data corruption in memory before writing to disk), or even interference from other processes like aggressive antivirus scans locking files during writes. Regardless of the specific trigger, the outcome is one or more cache files that are no longer valid SSIS raw data files, blocking the upload process.

Resolution: Identifying and Clearing Corrupted Cache Files¶

The resolution for Data Collector upload failures caused by corrupted cache files involves locating and removing the problematic files. Since the SSIS upload task fails when it encounters any corrupted file it needs to process, clearing the corrupted files allows the upload process to resume with the remaining valid cache files.

Here is a step-by-step guide to resolve this issue:

1. Connect to the SQL Server Instance: Use SQL Server Management Studio (SSMS) to connect to the specific instance of SQL Server where the Data Collector error is occurring and the collection sets are configured. Ensure you have appropriate permissions to manage Data Collector and access file system paths.

2. Access Data Collection Properties: In SSMS, expand the Management folder in the Object Explorer. Right-click on Data Collection and select Properties from the context menu. This will open the Data Collection Properties dialog box.

3. Identify the Cache Directory Location: In the Data Collection Properties dialog box, examine the Cache directory field.

   • If a specific directory path is listed in this field, this is the custom location configured for Data Collector’s cache files. Note down this path. You will need to navigate to this directory on the server’s file system.
   • If the Cache directory field is empty, it means Data Collector is using the default cache location. The default location is the local temporary directory of the Windows account that executes the collection set.

4. Determine the Default Cache Directory (if applicable): If the cache directory is not explicitly set, you need to identify the account running the collection set and find its temporary directory.

   • Collection sets are typically run by SQL Server Agent jobs. The account running the job is the SQL Server Agent service account. You can find this account by checking the SQL Server Configuration Manager or the properties of the SQL Server Agent service in the Windows Services console.
   • Once you know the service account name (e.g., DOMAIN\SQLAgentAccount or a local system account like NT Service\SQLSERVERAGENT), you need to find its temporary directory. For most service accounts on modern Windows Server versions, the temporary directory is typically located under C:\Users\<AccountName>\AppData\Local\Temp or a similar path within the user profile directory. For built-in accounts like NT Service\SQLSERVERAGENT, the path might be within the system’s profile structure, like C:\Windows\ServiceProfiles\NtService\AppData\Local\Temp or similar variations depending on the specific service name and OS version. It’s crucial to find the actual temporary directory used by that account’s process.

5. Locate and Isolate Cache Files: Navigate to the identified cache directory using Windows Explorer or a command prompt. Look for all files with a *.CACHE file name extension. These are the Data Collector cache files. To resolve the issue, you need to move these files out of the cache directory. Create a temporary subdirectory elsewhere on the disk (e.g., C:\temp\DCCacheBackup) and move all *.CACHE files from the cache directory to this backup location. Do not delete the files immediately. Moving them preserves them in case there’s a need for further investigation or if you move files that were not actually corrupted (though any file present during the failure is suspect).

6. Restart or Wait for Collection Sets:

   • For the Utility Information collection set: This is a built-in system collection set used for the Central Management Server (CMS) feature. It cannot be directly stopped and restarted like custom collection sets. After clearing the cache files, you must wait for the next scheduled collection and upload cycle to see if the problem is resolved. This cycle typically runs every 30 minutes. Monitor the collection set logs after this period.
   • For all other collection sets (System Data collection set, Custom collection sets): You can manually stop and restart these collection sets. In SSMS, navigate to the Data Collection node, expand Collection Sets, right-click the affected collection set, and select Stop Collection Set. Once stopped, right-click again and select Start Collection Set. This will force the collection set to begin a new cycle, using a fresh set of cache files.

7. Verify Resolution: After restarting the collection sets or waiting for the Utility Information collection set’s next cycle, monitor the collection set logs again. Check for new log entries and confirm that the upload task (often named something like MDW_upload_job_...) is now completing successfully without the previous error messages. Also, check the Management Data Warehouse database to see if new data is being uploaded for the affected collection set.

By isolating the corrupted cache files, you effectively remove the obstacle that was preventing the SSIS upload package from running successfully. The Data Collector will generate new, clean cache files for subsequent collection cycles.

More Information on Data Collector Operations¶

Understanding the broader context of how Data Collector operates can aid in both troubleshooting and preventing issues like cache file corruption. Data Collector is built upon several SQL Server technologies, including SQL Server Agent, SSIS, and the Management Data Warehouse database.

A collection set is the fundamental unit of data collection. It contains one or more collection items, which define what data to collect, how often to collect it, and what collector type to use (e.g., Generic T-SQL Query, SQL Trace, Performance Counters). When a collection set runs, it executes its collection items according to the defined schedule.

The collected data is then written to the cache directory. Data Collector uses a buffered writing mechanism to store this data in the *.CACHE files. This process must complete successfully to ensure the integrity of the cache file. Issues during this phase are the direct cause of the corruption.

Scheduled upload jobs (executed by SQL Server Agent) are responsible for reading the data from the cache files and uploading it to the MDW. These jobs utilize SSIS packages dynamically generated by Data Collector. The SSIS package reads the raw data files from the cache directory using an SSIS Raw File Source component and loads the data into the appropriate tables in the MDW using OLE DB Destination or similar components. If the Raw File Source encounters a corrupted file (as indicated by the error messages), the entire upload task for that batch of files fails.

Preventing Future Corruption¶

Preventing cache file corruption primarily involves ensuring a stable environment for the Data Collector process and the storage subsystem it uses. Key preventative measures include:

• Disk Space Monitoring: Implement robust monitoring for the disk volume hosting the Data Collector cache directory. Set up alerts to be notified when free space falls below a critical threshold. Proactively manage disk space on this volume.
• System Stability: Address underlying system stability issues, such as frequent unexpected server reboots, application crashes, or resource contention (CPU, memory) that could cause the SQL Server or SQL Server Agent processes to terminate abruptly during collection cycles.
• Storage Health: Ensure the underlying storage hardware (HDDs, SSDs, RAID controllers) is healthy and performing correctly. Keep storage drivers and firmware updated to the latest stable versions recommended by the hardware vendor. Perform regular checks of disk health using utilities like chkdsk or vendor-specific tools.
• Resource Management: Ensure the server hosting SQL Server and Data Collector has sufficient resources (CPU, RAM) to handle the workload, especially during peak collection times. Resource exhaustion can lead to processes being terminated or becoming unresponsive, potentially interrupting file write operations.
• Antivirus Exclusions: Configure antivirus software running on the SQL Server to exclude the Data Collector cache directory from real-time scanning. Antivirus locking files during writes can potentially interfere with the Data Collector’s ability to write data correctly.
• Review Collection Sets: Ensure collection sets are not configured to collect excessive amounts of data that could overwhelm disk I/O or quickly fill up the cache directory. Review the frequency and scope of collection items.

Troubleshooting Data Collector Beyond Cache Issues¶

While cache corruption is a specific issue, Data Collector can fail for other reasons. If clearing cache files does not resolve upload failures, consider these troubleshooting steps:

• SQL Server Agent Job Status: Check the history of the Data Collector upload jobs in SQL Server Agent. Are the jobs running? Are they failing with different errors?
• MDW Database Connectivity and Permissions: Verify that the SQL Server Agent account has the necessary permissions to connect to the MDW database and write data to its tables. Check network connectivity between the SQL Server instance running Data Collector and the instance hosting the MDW.
• MDW Database Health: Ensure the MDW database itself is online, healthy, and has sufficient space. Database issues can prevent successful data uploads.
• SSIS Catalog and Environment: If the MDW is hosted on SQL Server 2012 or later and uses the SSIS Catalog, check the execution reports for the Data Collector upload packages. These reports can provide more detailed SSIS-specific error information.
• SQL Server and Agent Error Logs: Review the SQL Server and SQL Server Agent error logs for any messages related to Data Collector, SSIS, or job execution that coincide with the upload failures.

Resolving Data Collector issues requires a systematic approach, checking configuration, permissions, resource availability, and the health of dependent components like SQL Server Agent, SSIS, and the MDW.

Data Collector plays a vital role in monitoring SQL Server environments. Maintaining its health, including the integrity of its cache files, is essential for reliable performance data collection and analysis.

Have you encountered Data Collector cache file corruption or other upload issues? Share your experiences and troubleshooting tips in the comments below!