Configuring WinCC Tag Logging Fast and Slow Archive Folders in WinCC RT Professional

WinCC Runtime archives tag values into two parallel storage tiers: the Tag Logging Fast database and the Tag Logging Slow database. Even when every configured logging cycle is sub-minute and all tags appear to be written to the fast tier, both folders and their underlying SQL Server segments are still provisioned on disk. This is a deliberate design constraint, not a misconfiguration. The sections below explain the underlying architecture, the segment rules that govern both folders, and the exact runtime parameters that control their size and retention.

1. Tag Logging Architecture Overview

WinCC (TIA Portal V20 / WinCC RT Professional and WinCC V7.x) separates process value archiving into two distinct log types to balance acquisition resolution against long-term retention cost:

Both databases are created automatically the first time Runtime is started. The folder presence on disk is therefore not a reliable indicator of which tier your tags are actually using — only the segment content tells you that.

2. Why Both Folders Always Appear

The WinCC archive subsystem instantiates the two database schemas as part of Runtime initialization. Once instantiated, the SQL Server service holds open file handles to *.mdf (primary data) and *.ldf (transaction log) pairs for both log types. Removing or renaming the slow folder while Runtime is active will cause the next tag-write attempt to fail and generate a database error in the WinCC diagnostics viewer.

Concretely, the slow folder is provisioned because:

1. The archive configuration object in the TIA Portal project always carries both fast and slow log definitions, even when only fast is used.
2. WinCC Runtime needs both schemas to exist so that operators can later re-assign tags to the slow tier without restarting the project.
3. The SQL Server instance creates the file group and transaction log on first attach, regardless of whether rows are written.

If you are certain that no tag will ever log to the slow tier, the only practical control available is to minimize the disk footprint of the slow folder — not to eliminate it.

3. Segment Rules and the 3-Segment Minimum

Each WinCC archive database is partitioned into time-based segments. A segment is a self-contained slice of the SQL database that can be swapped out (backed up, archived to network storage, or deleted) without affecting the active Runtime. Two parameters govern each segment:

WinCC enforces a hard minimum of 3 segments per log type. Even if you enter 1 or 2 in the project, Runtime will clamp the value to 3 at start-up. This is necessary because:

• The current segment cannot be closed while Runtime is writing to it.
• The previous segment may still be required for trend backfill queries that span the rollover boundary.
• A third segment is needed for the swap-in/swap-out cycle used by the online backup mechanism.

4. TIA Portal Configuration Path

In WinCC RT Professional (TIA Portal V17 to V20), the log sizes and segment parameters are defined under:

Runtime settings > Logging

Refer to the official TIA Portal documentation page Defining the Log Size and Segmentation (RT Professional) — TIA Portal V20 for the full dialog reference. The dialog exposes the following controls:

4.1 Minimizing the Slow Folder

If you do not write any data to the slow tier, the .mdf file will remain at its initial allocation size (typically 8 MB for the primary data file plus 8 MB for the transaction log) and will not grow. The .ldf may grow slightly during the first attach and then stabilize. To keep the slow folder footprint at its theoretical minimum, configure the following in the Runtime settings > Logging dialog:

1. Set Time period of all segments for the slow data log to 1 day.
2. Leave Number of segments at the default (3) — it cannot be reduced further.
3. Accept the default archive path, or relocate it to a small dedicated volume if you want to isolate the unused slow data from your fast tier.

With these settings, the slow folder will consume roughly 24 to 48 MB of disk space and no more, regardless of how long Runtime runs.

4.2 Recommended Sizing for a Production Slow Tier

If you do use the slow tier for long-term trend retention, the sizing rule of thumb is:

Slow footprint (MB) ≈ (Sum of slow-tag acquisition cycles per day / 86 400) × bytes per sample × days retained × 1.3

Where bytes per sample is typically 24 to 32 bytes (8-byte timestamp + 8-byte value + 8-byte quality + 8-byte tag ID overhead). For a 1 000-tag slow archive with a 1-minute cycle retained for 30 days, expect roughly 1.0 to 1.3 GB on disk.

5. SQL Server File Layout

Both WinCC archive databases are standard SQL Server (or SQL Server Express) databases. The *.mdf is the primary data file containing the compressed value tables, and the *.ldf is the transaction log used for rollback and online backup. The file naming convention is:

<ProjectPath>\Archive\TagLoggingFast\
    TagLoggingFast.mdf
    TagLoggingFast_log.ldf

<ProjectPath>\Archive\TagLoggingSlow\
    TagLoggingSlow.mdf
    TagLoggingSlow_log.ldf

Do not delete or move these files while the WinCC Runtime service is running. The SQL Server holds exclusive locks on the file handles. If you need to reclaim disk space, use the WinCC archive backup tool to export and remove the oldest closed segments, not the Windows file explorer.

6. How Tag Acquisition Cycle Maps to Log Type

The log type for a given tag is determined by the acquisition cycle configured on the tag's archive settings, not by the file path. A tag is routed to the fast tier if its cycle is in the fast-tier range, and to the slow tier otherwise. The default boundaries are:

The exact crossover threshold is project-specific and is set under Runtime settings > Logging > Boundary between fast and slow logging. If every tag in your project is set to a sub-minute cycle, all value rows will be written to the fast database, but the slow database schema and its three empty segments will still be created and remain on disk.

7. Runtime Diagnostics

To confirm which tier your tags are actually writing to, use the WinCC diagnostics tools:

1. Open the WinCC Explorer on the Runtime server and navigate to Diagnostics > Archive.
2. Check the Tag Logging Fast and Tag Logging Slow status tiles for write counters and last-write timestamps.
3. Cross-reference the ApDiag.out log file (default location: <ProjectPath>\Archive) for any archive subsystem errors.

If Tag Logging Slow shows zero writes over a 24-hour observation window, you can safely apply the minimum-footprint configuration described in section 4.1 without affecting trend or alarm data.

8. Troubleshooting Matrix

9. Performance Considerations

The fast tier is sized for high write throughput at the cost of compression ratio. The slow tier is sized for compression and long retention at the cost of write throughput. Mixing high-frequency tags into the slow tier is a common cause of archive backlogs and SQL Server transaction log growth.

Rule of thumb: keep the slow-tier tag count below 5 000 tags per WinCC server for a single-server deployment, and keep the fast-tier write rate below 5 000 samples per second sustained. Exceeding these values requires either segment size hints (to force earlier rollover) or a tiered archive strategy with an external historian.

10. Migration and Re-Initialization

If you need to reset the slow database to its initial empty state (for example, after a project clone in a test environment), the supported procedure is:

1. Stop the WinCC Runtime service.
2. Stop the SQL Server (WinCC) service.
3. Delete the contents of <ProjectPath>\Archive\TagLoggingSlow.
4. Start the SQL Server (WinCC) service.
5. Start the WinCC Runtime service — the slow database will be re-created with 3 empty segments.

The same procedure applies to the fast folder, but only do this on a test system. Re-initializing the fast database on a production line destroys all retained trend data.

11. Key Constraints Summary

• Both TagLoggingFast and TagLoggingSlow folders are always created when Runtime starts with archive configuration present.
• Folder presence does not indicate which tier is in use; check the WinCC diagnostics view for write counters.
• Each log type enforces a hard minimum of 3 segments.
• Minimum practical slow-tier footprint is 3 × 1-day segments.
• Do not delete *.mdf / *.ldf files while Runtime is active.
• Tag routing is determined by acquisition cycle, not by folder structure.

FAQ