azure: enhanced upload with single PutBlob API and configurable upload methods

internal/backend/azure/azure.go

@@ -42,6 +42,7 @@ type Backend struct {
 }
 
 const saveLargeSize = 256 * 1024 * 1024
+const singleBlobMaxSize = 5000 * 1024 * 1024 // 5000 MiB - max size for Put Blob API in service version 2019-12-12+
 const defaultListMaxItems = 5000
 
 // make sure that *Backend implements backend.Backend
@@ -53,6 +54,12 @@ func NewFactory() location.Factory {
 
 func open(cfg Config, rt http.RoundTripper) (*Backend, error) {
 	debug.Log("open, config %#v", cfg)
+
+	// Validate configuration
+	if err := cfg.Validate(); err != nil {
+		return nil, err
+	}
+
 	var client *azContainer.Client
 	var err error
 
@@ -255,14 +262,37 @@ func (be *Backend) Save(ctx context.Context, h backend.Handle, rd backend.Rewind
 	}
 
 	var err error
-	if rd.Length() < saveLargeSize {
-		// if it's smaller than 256miB, then just create the file directly from the reader
+	uploadMethod := strings.ToLower(be.cfg.UploadMethod)
+	fileSize := rd.Length()
+
+	switch uploadMethod {
+	case "single":
+		// Always use single blob upload
+		if fileSize > singleBlobMaxSize {
+			return errors.Errorf("file size %d exceeds single blob limit of %d MiB", fileSize, singleBlobMaxSize/1024/1024)
+		}
+		err = be.saveSingleBlob(ctx, objName, rd, accessTier)
+
+	case "blocks":
+		// Legacy block-based upload method
+		if fileSize < saveLargeSize {
 			err = be.saveSmall(ctx, objName, rd, accessTier)
 		} else {
-		// otherwise use the more complicated method
 			err = be.saveLarge(ctx, objName, rd, accessTier)
 		}
 
+	case "auto", "":
+		// Automatic selection: use single blob for files <= 5000 MiB, blocks for larger files
+		if fileSize <= singleBlobMaxSize {
+			err = be.saveSingleBlob(ctx, objName, rd, accessTier)
+		} else {
+			err = be.saveLarge(ctx, objName, rd, accessTier)
+		}
+
+	default:
+		return errors.Errorf("invalid upload method %q, must be 'auto', 'single', or 'blocks'", uploadMethod)
+	}
+
 	return err
 }
 
@@ -348,6 +378,29 @@ func (be *Backend) saveLarge(ctx context.Context, objName string, rd backend.Rew
 	return errors.Wrap(err, "CommitBlockList")
 }
 
+// saveSingleBlob uploads data using a single Put Blob operation.
+// This method is more efficient for files under 5000 MiB as it requires only one API call
+// instead of the two calls (StageBlock + CommitBlockList) required by the block-based approach.
+func (be *Backend) saveSingleBlob(ctx context.Context, objName string, rd backend.RewindReader, accessTier blob.AccessTier) error {
+	blockBlobClient := be.container.NewBlockBlobClient(objName)
+
+	buf := make([]byte, rd.Length())
+	_, err := io.ReadFull(rd, buf)
+	if err != nil {
+		return errors.Wrap(err, "ReadFull")
+	}
+
+	reader := bytes.NewReader(buf)
+	opts := &blockblob.UploadOptions{
+		Tier: &accessTier,
+		TransactionalValidation: blob.TransferValidationTypeMD5(rd.Hash()),
+	}
+
+	debug.Log("Upload single blob %v with %d bytes", objName, len(buf))
+	_, err = blockBlobClient.Upload(ctx, streaming.NopCloser(reader), opts)
+	return errors.Wrap(err, "Upload")
+}
+
 // Load runs fn with a reader that yields the contents of the file at h at the
 // given offset.
 func (be *Backend) Load(ctx context.Context, h backend.Handle, length int, offset int64, fn func(rd io.Reader) error) error {

internal/backend/azure/config.go

@@ -24,12 +24,14 @@ type Config struct {
 
 	Connections  uint   `option:"connections" help:"set a limit for the number of concurrent connections (default: 5)"`
 	AccessTier   string `option:"access-tier" help:"set the access tier for the blob storage (default: inferred from the storage account defaults)"`
+	UploadMethod string `option:"upload-method" help:"blob upload method: 'auto' (single blob for <=5000 MiB), 'single' (always single blob), or 'blocks' (legacy block-based) (default: auto)"`
 }
 
 // NewConfig returns a new Config with the default values filled in.
 func NewConfig() Config {
 	return Config{
 		Connections:  5,
+		UploadMethod: "auto",
 	}
 }
 
@@ -85,3 +87,16 @@ func (cfg *Config) ApplyEnvironment(prefix string) {
 		cfg.EndpointSuffix = os.Getenv(prefix + "AZURE_ENDPOINT_SUFFIX")
 	}
 }
+
+// Validate checks the configuration for errors.
+func (cfg *Config) Validate() error {
+	// Normalize upload method to lowercase
+	uploadMethod := strings.ToLower(cfg.UploadMethod)
+	if uploadMethod != "auto" && uploadMethod != "single" && uploadMethod != "blocks" && uploadMethod != "" {
+		return errors.Errorf("invalid upload method %q, must be 'auto', 'single', or 'blocks'", cfg.UploadMethod)
+	}
+	if uploadMethod != "" {
+		cfg.UploadMethod = uploadMethod
+	}
+	return nil
+}