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    Metadata preservation

    This document describes metadata that is preserved when you use Storage Transfer Service to transfer data between various sources and destinations.

    Overview

    Storage Transfer Service preserves the following metadata:

    • User-created custom metadata for transfers that originate from Cloud Storage, Amazon S3, or Microsoft Azure Blob Storage is preserved.

    • Transfers between Cloud Storage buckets can optionally preserve object ACLs, customer-managed encryption keys, storage class, object creation time (as the value of a customTime field), and temporary holds.

    • For transfers from any source to a Cloud Storage bucket, the object's storage class in the destination bucket can be set to any supported class as part of the transfer.

    • File size and last modified time ( mtime ) are preserved for transfers that originate from POSIX file systems. mtime is not preserved for folders.

    • Optionally, symlinks, numeric UID, numeric GID, and numeric MODE can be preserved for transfers to and from POSIX file systems.

    • For transfers between file systems only, if UID, GID, or MODE are preserved, that metadata is also preserved for folders. Cloud Storage recreates folders on the destination file system and restores UID, GID, and/or MODE. This includes empty folders. mtime is not preserved.

      Folder-level metadata is not preserved if the transfer is by manifest .

    Metadata fields that are not explicitly mentioned in this document are not preserved.

    Metadata preservation behavior

    The following sections list metadata examples from different source storage systems and how Storage Transfer Service preserves metadata from each. For an exhaustive list of metadata, refer to the source storage system's documentation.

    Amazon S3 or S3-compatible storage to Cloud Storage

    Metadata example
    Preservation behavior
    Amazon S3 fixed-key metadata fields, such as: Cache-Control , Content-Disposition , and Content-Type .
    Preserved as fixed-key metadata.
    Amazon S3 user-defined metadata, formatted as key:value pairs. For more information, see the User-defined object metadata section of Object key and metadata .

    Preserved as a custom metadata field in destination Cloud Storage objects, which you can edit later or remove.

    ETag
    Preserved as a custom metadata field with the key x-goog-source-etag , which you can edit later or remove.
    Object size.
    Preserved as size .
    Amazon S3 access control lists (ACLs). For a complete list, see the Condition Keys section of Access Control List (ACL) Overview .
    Not preserved.
    Amazon S3 object tags, defined by you as key-value pairs. For more information, see Object Tags .
    Not preserved.
    Amazon S3 system-defined metadata, except for ETag and object size. For a complete list, see the System-defined object metadata section of Object key and metadata .

    Not preserved.

    Timestamp metadata from the source isn't preserved. Creation time, timeCreated , reflects the time that an object is created in Cloud Storage. Similarly, updated reflects the time that metadata for an object is modified in Cloud Storage.

    Storage class

    There are multiple options for setting storage class during a transfer.

    • Set each object's storage class to that of the destination bucket. This is the default behavior.
    • Set a specific storage class on all objects being transferred.

    See the metadataOptions reference documentation for details.

    Microsoft Azure Storage to Cloud Storage

    Metadata example
    Preservation behavior
    Microsoft Azure Storage fixed-key metadata fields, such as: Cache-Control , Content-Disposition , and Content-Type .
    Preserved as fixed-key metadata.
    Microsoft Azure Storage user-defined metadata, formatted as key:value pairs. For more information, see Settings and retrieving properties and metadata for Blob service resources .

    Preserved as a custom metadata field in destination Cloud Storage objects, which you can edit later or remove.

    ETag
    Preserved as a custom metadata field with the key x-goog-source-etag , which you can edit later or remove.
    Object size.
    Preserved as size .
    POSIX file system permissions supported by Azure Data Lake Storage (ADLS) Gen 2.
    Not preserved.
    Microsoft Azure Storage access control, specifically x-ms-blob-public-access . For more information, see the Response Headers section of Get Container ACL .
    Not preserved.
    Microsoft Azure Storage index tags. For more information, see Manage and find Azure Blob data with blob index tags .
    Not preserved.
    Microsoft Azure Storage timestamp metadata, such as: Last-Modified , x-ms-creation-time , x-ms-version , x-ms-request-server-encrypted , and x-ms-encryption-scope . For more information, see Set Blob Metadata .

    Not preserved.

    Timestamp metadata from the source isn't preserved. Creation time, timeCreated , reflects the time that an object is created in Cloud Storage. Similarly, updated reflects the time that metadata for an object is modified in Cloud Storage.

    Storage class

    There are multiple options for setting storage class during a transfer.

    • Set each object's storage class to that of the destination bucket. This is the default behavior.
    • Set a specific storage class on all objects being transferred.

    See the metadataOptions reference documentation for details.

    Transfers between Cloud Storage buckets

    Metadata example
    Preservation behavior

    Cloud Storage fixed-key metadata fields, such as: Cache-Control , Content-Disposition , and Content-Type .

    For more information, see Object metadata

    Preserved as fixed-key metadata.

    Cloud Storage user-defined metadata, formatted as key:value pairs. For more information, see Custom metadata .

    Preserved as a custom metadata field in destination Cloud Storage objects, which you can edit later or remove.

    Object size
    Preserved as size .
    Object generation
    Preserved as a custom metadata field with the key x-goog-reserved-source-generation , which you can edit later or remove.
    Object holds

    Event-based holds are not preserved. If the destination bucket has the default event-based hold property enabled, an event-based hold is applied to transferred objects.

    Temporary holds are preserved by default. To discard temporary holds during the transfer, set the temporaryHold field of the metadataOptions object to TEMPORARY_HOLD_SKIP .

    Access Control Lists (ACLs)

    ACLs can optionally be preserved. See the metadataOptions reference documentation for details.

    When preserving ACLs be careful to avoid creating inaccessible objects .

    For more information, see the Cloud Storage Access Control Lists documentation.

    Storage class

    There are multiple options for setting storage class during a transfer.

    • Set each object's storage class to that of the destination bucket. This is the default behavior.
    • Preserve the source object's storage class.
    • Set a specific storage class on all objects being transferred.

    See the metadataOptions reference documentation for details.

    Customer-managed encryption key

    If a customer-managed encryption key (CMEK) is being used on an object, the object can optionally use the same key when it is written to the destination bucket.

    The default behavior is to write the object to the destination bucket using the bucket's encryption method.

    When preserving the original CMEK, be aware of the following limitations:

    See the metadataOptions reference documentation for details.

    Timestamp metadata

    timeCreated can optionally be preserved. The preserved value is stored in the customTime field of the transferred object in Cloud Storage. See the metadataOptions reference documentation for details.

    updated metadata is not preserved.

    Other Cloud Storage non-editable metadata, such as etag and componentCount .
    Not Preserved.

    For a list of metadata in Cloud Storage, see Objects .

    URL list transfer to Cloud Storage

    For more information about URL lists, see Creating a URL list .

    Metadata example
    Preservation behavior
    Fixed-key metadata fields, such as: Cache-Control , Content-Disposition , and Content-Type .
    Preserved as editable metadata.
    Content-Length and MD5

    Preserved as non-editable metadata.

    If the source doesn't provide an MD5 hash value, then we don't preserve a value.

    This preservation behavior is specific to Content-Length and MD5 . Any other non-editable metadata not listed is not preserved.

    Timestamp metadata, such as: Creation time, modified time, and other source-specific metadata.

    Not preserved.

    Timestamp metadata from the source isn't preserved. Creation time, timeCreated , reflects the time that an object is created in Cloud Storage. Similarly, updated reflects the time that metadata for an object is modified in Cloud Storage.

    Storage class

    There are multiple options for setting storage class during a transfer.

    • Set each object's storage class to that of the destination bucket. This is the default behavior.
    • Set a specific storage class on all objects being transferred.

    See the metadataOptions reference documentation for details.

    POSIX file system transfers

    When transferring files from POSIX file systems, Storage Transfer Service can optionally preserve certain attributes as custom metadata. If these files are later written back to a file system, Storage Transfer Service can convert the preserved metadata back to POSIX attributes.

    Metadata example
    Preservation behavior
    Modified time ( mtime )

    Preserved.

    mtime is preserved as custom metadata with the key goog-reserved-file-mtime .

    File size

    Preserved.

    File size is preserved as size .

    Numeric UID
    Numeric GID
    Numeric MODE
    Symbolic links

    Optional.

    Preservation behavior is specified with the metadataOptions object. See Preserving optional POSIX metadata for details.

    Default behavior is to not preserve any metadata.

    Folder metadata
    Folder-level metadata is only preserved for transfers between file systems. The transfer's UID, GID, and MODE preservation settings apply to files and folders for these transfers.

    mtime is not preserved for folders. mtime is set to the create time of the folder on the destination file system.

    Folder metadata is not preserved for manifest transfers .

    Storage class

    There are multiple options for setting storage class during a transfer.

    • Set each object's storage class to that of the destination bucket. This is the default behavior.
    • Set a specific storage class on all objects being transferred.

    See the metadataOptions reference documentation for details.

    Preserving optional POSIX metadata

    To preserve one or more of numeric UID, numeric GID, numeric MODE, and symbolic links, specify a metadataOptions object in the body of your transfer job.

    These options apply to both POSIX-to-Cloud Storage transfers and Cloud Storage-to-POSIX transfers. For the latter, the metadata must have been preserved when files were initially transferred to Cloud Storage.

     {
  "description": "metadata-example",
  "projectId": "example-project-id"
  "transferSpec": {
    ...
    "transferOptions": {
      "metadataOptions": {
        "gid":     "GID_NUMBER",       # Default is "GID_SKIP"
        "uid":     "UID_NUMBER",       # Default is "UID_SKIP"
        "mode":    "MODE_PRESERVE",    # Default is "MODE_SKIP"
        "symlink": "SYMLINK_PRESERVE"  # Default is "SYMLINK_SKIP"
      }
    }
  }
}

    POSIX to Cloud Storage

    Preserved metadata is stored in Cloud Storage as custom metadata key:value pairs.

    • Numeric GID is stored as goog-reserved-posix-gid .
    • Numeric UID is stored as goog-reserved-posix-uid .
    • Numeric MODE is stored as goog-reserved-posix-mode .

    For symbolic links, Storage Transfer Service preserves the target link as an object in Cloud Storage with the following qualities:

    • Object key is composed of the destination prefix plus the path to the symlink, relative to the root_directory .
    • Object metadata:
      • Any symlink metadata is preserved as Cloud Storage object metadata.
      • A custom metadata entry is made: goog-reserved-file-is-symlink:true .
    • The object content is the target of the symlink. For example, for a symlink sym-> dir1/target , the object's content is "dir1/target".

    Storage Transfer Service does not validate the link or copy the target file.

    Cloud Storage to POSIX

    If metadata is preserved when files are transferred to Cloud Storage, that metadata can be written back to the files when they are transferred back to a POSIX file system .

    If a metadata option is set to preserve, Storage Transfer Service takes the following actions:

    • Symbolic links: Storage Transfer Service creates a symlink file pointing to the target link. If the target file does not exist, the symlink will be broken.
    • GID, UID, and MODE: the values stored in Cloud Storage metadata are written back to the file.

    POSIX to POSIX

    Transfers between file systems can optionally preserve GID, UID, and MODE for files and folders.

    Last modified time is saved for files, but not for folders. mtime is set to the create time of the folder on the destination file system.

    Storage Transfer Service saves folder metadata by creating 0-byte folder objects in the intermediate bucket, then copying that metadata back to the folder on the destination file system. For this reason, the number of objects created in the intermediate bucket may be greater than the number of files being transferred.
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