Data columns cache warmup: Parallelize computation of all files for a given epoch. (#16207)

**What type of PR is this?**
Other

**What does this PR do? Why is it needed?**
Before this PR, all `.sszs` files containing the data column sidecars
were read an process sequentially, taking some time.
After this PR, every `.sszs` files of a given epoch (so, up to 32 files
with the current `SLOT_PER_EPOCHS` value) are processed in parallel.

**Which issues(s) does this PR fix?**
- https://github.com/OffchainLabs/prysm/issues/16204

Tested on - [Netcup VPS 4000 G11](https://www.netcup.com/en/server/vps).
**Before this PR (3 trials)**:
```
[2026-01-02 08:55:12.71]  INFO filesystem: Data column filesystem cache warm-up complete elapsed=1m22.894007534s
[2026-01-02 12:59:33.62]  INFO filesystem: Data column filesystem cache warm-up complete elapsed=42.346732863s
[2026-01-02 13:03:13.65]  INFO filesystem: Data column filesystem cache warm-up complete elapsed=56.143565960s
```

**After this PR (3 trials)**:
```
[2026-01-02 12:50:07.53]  INFO filesystem: Data column filesystem cache warm-up complete elapsed=2.019424193s
[2026-01-02 12:52:01.34]  INFO filesystem: Data column filesystem cache warm-up complete elapsed=1.960671225s
[2026-01-02 12:53:34.66]  INFO filesystem: Data column filesystem cache warm-up complete elapsed=2.549555363s
```


**Acknowledgements**
- [x] I have read
[CONTRIBUTING.md](https://github.com/prysmaticlabs/prysm/blob/develop/CONTRIBUTING.md).
- [x] I have included a uniquely named [changelog fragment
file](https://github.com/prysmaticlabs/prysm/blob/develop/CONTRIBUTING.md#maintaining-changelogmd).
- [x] I have added a description with sufficient context for reviewers
to understand this PR.
- [x] I have tested that my changes work as expected and I added a
testing plan to the PR description (if applicable).

beacon-chain/db/filesystem/BUILD.bazel

@@ -38,6 +38,7 @@ go_library(
         "@com_github_prometheus_client_golang//prometheus/promauto:go_default_library",
         "@com_github_sirupsen_logrus//:go_default_library",
         "@com_github_spf13_afero//:go_default_library",
+        "@org_golang_x_sync//errgroup:go_default_library",
     ],
 )
 

beacon-chain/db/filesystem/data_column.go

@@ -25,6 +25,7 @@ import (
 	"github.com/OffchainLabs/prysm/v7/time/slots"
 	"github.com/pkg/errors"
 	"github.com/spf13/afero"
+	"golang.org/x/sync/errgroup"
 )
 
 const (
@@ -185,73 +186,162 @@ func (dcs *DataColumnStorage) WarmCache() {
 
 	highestStoredEpoch := primitives.Epoch(0)
 
-	// Walk the data column filesystem to warm up the cache.
-	if err := afero.Walk(dcs.fs, ".", func(path string, info os.FileInfo, fileErr error) (err error) {
-		if fileErr != nil {
-			return fileErr
-		}
-
-		// If not a leaf, skip.
-		if info.IsDir() {
-			return nil
-		}
-
-		// Extract metadata from the file path.
-		fileMetadata, err := extractFileMetadata(path)
-		if err != nil {
-			log.WithError(err).Error("Error encountered while extracting file metadata")
-			return nil
-		}
-
-		// Open the data column filesystem file.
-		f, err := dcs.fs.Open(path)
-		if err != nil {
-			log.WithError(err).Error("Error encountered while opening data column filesystem file")
-			return nil
-		}
-
-		// Close the file.
-		defer func() {
-			// Overwrite the existing error only if it is nil, since the close error is less important.
-			closeErr := f.Close()
-			if closeErr != nil && err == nil {
-				err = closeErr
-			}
-		}()
-
-		// Read the metadata of the file.
-		metadata, err := dcs.metadata(f)
-		if err != nil {
-			log.WithError(err).Error("Error encountered while reading metadata from data column filesystem file")
-			return nil
-		}
-
-		// Check the indices.
-		indices := metadata.indices.all()
-		if len(indices) == 0 {
-			return nil
-		}
-
-		// Build the ident.
-		dataColumnsIdent := DataColumnsIdent{Root: fileMetadata.blockRoot, Epoch: fileMetadata.epoch, Indices: indices}
-
-		// Update the highest stored epoch.
-		highestStoredEpoch = max(highestStoredEpoch, fileMetadata.epoch)
-
-		// Set the ident in the cache.
-		if err := dcs.cache.set(dataColumnsIdent); err != nil {
-			log.WithError(err).Error("Error encountered while ensuring data column filesystem cache")
-		}
-
-		return nil
-	}); err != nil {
-		log.WithError(err).Error("Error encountered while walking data column filesystem.")
+	// List all period directories
+	periodFileInfos, err := afero.ReadDir(dcs.fs, ".")
+	if err != nil {
+		log.WithError(err).Error("Error reading top directory during warm cache")
+		return
 	}
 
-	// Prune the cache and the filesystem.
+	// Iterate through periods
+	for _, periodFileInfo := range periodFileInfos {
+		if !periodFileInfo.IsDir() {
+			continue
+		}
+
+		periodPath := periodFileInfo.Name()
+
+		// List all epoch directories in this period
+		epochFileInfos, err := afero.ReadDir(dcs.fs, periodPath)
+		if err != nil {
+			log.WithError(err).WithField("period", periodPath).Error("Error reading period directory during warm cache")
+			continue
+		}
+
+		// Iterate through epochs
+		for _, epochFileInfo := range epochFileInfos {
+			if !epochFileInfo.IsDir() {
+				continue
+			}
+
+			epochPath := path.Join(periodPath, epochFileInfo.Name())
+
+			// List all .sszs files in this epoch
+			files, err := listEpochFiles(dcs.fs, epochPath)
+			if err != nil {
+				log.WithError(err).WithField("epoch", epochPath).Error("Error listing epoch files during warm cache")
+				continue
+			}
+
+			if len(files) == 0 {
+				continue
+			}
+
+			// Process all files in this epoch in parallel
+			epochHighest, err := dcs.processEpochFiles(files)
+			if err != nil {
+				log.WithError(err).WithField("epoch", epochPath).Error("Error processing epoch files during warm cache")
+			}
+
+			highestStoredEpoch = max(highestStoredEpoch, epochHighest)
+		}
+	}
+
+	// Prune the cache and the filesystem
 	dcs.prune()
 
-	log.WithField("elapsed", time.Since(start)).Info("Data column filesystem cache warm-up complete")
+	totalElapsed := time.Since(start)
+
+	// Log summary
+	log.WithField("elapsed", totalElapsed).Info("Data column filesystem cache warm-up complete")
+}
+
+// listEpochFiles lists all .sszs files in an epoch directory.
+func listEpochFiles(fs afero.Fs, epochPath string) ([]string, error) {
+	fileInfos, err := afero.ReadDir(fs, epochPath)
+	if err != nil {
+		return nil, errors.Wrap(err, "read epoch directory")
+	}
+
+	files := make([]string, 0, len(fileInfos))
+	for _, fileInfo := range fileInfos {
+		if fileInfo.IsDir() {
+			continue
+		}
+
+		fileName := fileInfo.Name()
+		if strings.HasSuffix(fileName, "."+dataColumnsFileExtension) {
+			files = append(files, path.Join(epochPath, fileName))
+		}
+	}
+
+	return files, nil
+}
+
+// processEpochFiles processes all .sszs files in an epoch directory in parallel.
+func (dcs *DataColumnStorage) processEpochFiles(files []string) (primitives.Epoch, error) {
+	var (
+		eg errgroup.Group
+		mu sync.Mutex
+	)
+
+	highestEpoch := primitives.Epoch(0)
+	for _, filePath := range files {
+		eg.Go(func() error {
+			epoch, err := dcs.processFile(filePath)
+			if err != nil {
+				log.WithError(err).WithField("file", filePath).Error("Error processing file during warm cache")
+				return nil
+			}
+
+			mu.Lock()
+			defer mu.Unlock()
+			highestEpoch = max(highestEpoch, epoch)
+
+			return nil
+		})
+	}
+
+	if err := eg.Wait(); err != nil {
+		return highestEpoch, err
+	}
+
+	return highestEpoch, nil
+}
+
+// processFile processes a single .sszs file.
+func (dcs *DataColumnStorage) processFile(filePath string) (primitives.Epoch, error) {
+	// Extract metadata from the file path
+	fileMetadata, err := extractFileMetadata(filePath)
+	if err != nil {
+		return 0, errors.Wrap(err, "extract file metadata")
+	}
+
+	// Open the file (each goroutine gets its own FD)
+	f, err := dcs.fs.Open(filePath)
+	if err != nil {
+		return 0, errors.Wrap(err, "open file")
+	}
+	defer func() {
+		if closeErr := f.Close(); closeErr != nil {
+			log.WithError(closeErr).WithField("file", filePath).Error("Error closing file during warm cache")
+		}
+	}()
+
+	// Read metadata
+	metadata, err := dcs.metadata(f)
+	if err != nil {
+		return 0, errors.Wrap(err, "read metadata")
+	}
+
+	// Extract indices
+	indices := metadata.indices.all()
+	if len(indices) == 0 {
+		return fileMetadata.epoch, nil // No indices, skip
+	}
+
+	// Build ident and set in cache (thread-safe)
+	dataColumnsIdent := DataColumnsIdent{
+		Root:    fileMetadata.blockRoot,
+		Epoch:   fileMetadata.epoch,
+		Indices: indices,
+	}
+
+	if err := dcs.cache.set(dataColumnsIdent); err != nil {
+		return 0, errors.Wrap(err, "cache set")
+	}
+
+	return fileMetadata.epoch, nil
 }
 
 // Summary returns the DataColumnStorageSummary.

changelog/manu-cache-warmup.md

@@ -0,0 +1,3 @@
+### Changed
+
+- Data column sidecars cache warmup: Process in parallel all sidecars for a given epoch.