azure: add support for hot, cool, or cool access tiers

changelog/unreleased/issue-4521

@@ -0,0 +1,21 @@
+Enhancement: Add config option to set Microsoft Blob Storage Access Tier
+
+The `azure.access-tier` option can be passed to Restic (using `-o`) to
+specify the access tier for Microsoft Blob Storage objects created by Restic.
+
+The access tier is passed as-is to Microsoft Blob Storage, so it needs to be
+understood by the API. The allowed values are `Hot`, `Cool`, or `Cold`.
+
+If unspecified, the default is inferred from the default configured on the 
+storage account.
+
+You can mix access tiers in the same container, and the setting isn't
+stored in the restic repository, so be sure to specify it with each
+command that writes to Microsoft Blob Storage.
+
+There is no official `Archive` storage support in restic, use this option at
+your own risk. To restore any data, it is still necessary to manually warm up
+the required data in the `Archive` tier.
+
+https://github.com/restic/restic/issues/4521
+https://github.com/restic/restic/pull/5046

doc/030_preparing_a_new_repo.rst

@@ -568,6 +568,10 @@ The number of concurrent connections to the Azure Blob Storage service can be se
 ``-o azure.connections=10`` switch. By default, at most five parallel connections are
 established.
 
+The access tier of the blobs uploaded to the Azure Blob Storage service can be set with the
+``-o azure.access-tier=Cool`` switch. The allowed values are ``Hot``, ``Cool`` or ``Cold``. 
+If unspecified, the default is inferred from the default configured on the storage account.
+
 Google Cloud Storage
 ********************
 

internal/backend/azure/azure.go

@@ -37,6 +37,8 @@ type Backend struct {
 	prefix       string
 	listMaxItems int
 	layout.Layout
+
+	accessTier blob.AccessTier
 }
 
 const saveLargeSize = 256 * 1024 * 1024
@@ -124,17 +126,32 @@ func open(cfg Config, rt http.RoundTripper) (*Backend, error) {
 		}
 	}
 
+	var accessTier blob.AccessTier
+	// if the access tier is not supported, then we will not set the access tier; during the upload process,
+	// the value will be inferred from the default configured on the storage account.
+	for _, tier := range supportedAccessTiers() {
+		if strings.EqualFold(string(tier), cfg.AccessTier) {
+			accessTier = tier
+			break
+		}
+	}
+
 	be := &Backend{
 		container:    client,
 		cfg:          cfg,
 		connections:  cfg.Connections,
 		Layout:       layout.NewDefaultLayout(cfg.Prefix, path.Join),
 		listMaxItems: defaultListMaxItems,
+		accessTier:   accessTier,
 	}
 
 	return be, nil
 }
 
+func supportedAccessTiers() []blob.AccessTier {
+	return []blob.AccessTier{blob.AccessTierHot, blob.AccessTierCool, blob.AccessTierCold, blob.AccessTierArchive}
+}
+
 // Open opens the Azure backend at specified container.
 func Open(_ context.Context, cfg Config, rt http.RoundTripper) (*Backend, error) {
 	return open(cfg, rt)
@@ -213,25 +230,39 @@ func (be *Backend) Path() string {
 	return be.prefix
 }
 
+// useAccessTier determines whether to apply the configured access tier to a given file.
+// For archive access tier, only data files are stored using that class; metadata
+// must remain instantly accessible.
+func (be *Backend) useAccessTier(h backend.Handle) bool {
+	notArchiveClass := !strings.EqualFold(be.cfg.AccessTier, "archive")
+	isDataFile := h.Type == backend.PackFile && !h.IsMetadata
+	return isDataFile || notArchiveClass
+}
+
 // Save stores data in the backend at the handle.
 func (be *Backend) Save(ctx context.Context, h backend.Handle, rd backend.RewindReader) error {
 	objName := be.Filename(h)
 
 	debug.Log("InsertObject(%v, %v)", be.cfg.AccountName, objName)
 
+	var accessTier blob.AccessTier
+	if be.useAccessTier(h) {
+		accessTier = be.accessTier
+	}
+
 	var err error
 	if rd.Length() < saveLargeSize {
 		// if it's smaller than 256miB, then just create the file directly from the reader
-		err = be.saveSmall(ctx, objName, rd)
+		err = be.saveSmall(ctx, objName, rd, accessTier)
 	} else {
 		// otherwise use the more complicated method
-		err = be.saveLarge(ctx, objName, rd)
+		err = be.saveLarge(ctx, objName, rd, accessTier)
 	}
 
 	return err
 }
 
-func (be *Backend) saveSmall(ctx context.Context, objName string, rd backend.RewindReader) error {
+func (be *Backend) saveSmall(ctx context.Context, objName string, rd backend.RewindReader, accessTier blob.AccessTier) error {
 	blockBlobClient := be.container.NewBlockBlobClient(objName)
 
 	// upload it as a new "block", use the base64 hash for the ID
@@ -252,11 +283,13 @@ func (be *Backend) saveSmall(ctx context.Context, objName string, rd backend.Rew
 	}
 
 	blocks := []string{id}
-	_, err = blockBlobClient.CommitBlockList(ctx, blocks, &blockblob.CommitBlockListOptions{})
+	_, err = blockBlobClient.CommitBlockList(ctx, blocks, &blockblob.CommitBlockListOptions{
+		Tier: &accessTier,
+	})
 	return errors.Wrap(err, "CommitBlockList")
 }
 
-func (be *Backend) saveLarge(ctx context.Context, objName string, rd backend.RewindReader) error {
+func (be *Backend) saveLarge(ctx context.Context, objName string, rd backend.RewindReader, accessTier blob.AccessTier) error {
 	blockBlobClient := be.container.NewBlockBlobClient(objName)
 
 	buf := make([]byte, 100*1024*1024)
@@ -303,7 +336,9 @@ func (be *Backend) saveLarge(ctx context.Context, objName string, rd backend.Rew
 		return errors.Errorf("wrote %d bytes instead of the expected %d bytes", uploadedBytes, rd.Length())
 	}
 
-	_, err := blockBlobClient.CommitBlockList(ctx, blocks, &blockblob.CommitBlockListOptions{})
+	_, err := blockBlobClient.CommitBlockList(ctx, blocks, &blockblob.CommitBlockListOptions{
+		Tier: &accessTier,
+	})
 
 	debug.Log("uploaded %d parts: %v", len(blocks), blocks)
 	return errors.Wrap(err, "CommitBlockList")

internal/backend/azure/config.go

@@ -22,7 +22,8 @@ type Config struct {
 	Container          string
 	Prefix             string
 
-	Connections uint `option:"connections" help:"set a limit for the number of concurrent connections (default: 5)"`
+	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)"`
 }
 
 // NewConfig returns a new Config with the default values filled in.