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 during a transfer to Cloud Storage:

User-created custom metadata for transfers that originate from Cloud Storage, Amazon Simple Storage Service (Amazon S3), or Microsoft Azure Blob Storage (Microsoft Azure Storage).
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. 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) for transfers that originate from POSIX file systems.
Optionally, symlinks, numeric UID, numeric GID, and numeric MODE can be preserved for transfers from and to POSIX file systems.

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 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.

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.

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 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) For more information, see the Cloud Storage Access Control Lists documentation.	ACLs can optionally be preserved. See the metadataOptions reference documentation for details. When preserving ACLs be careful to avoid creating inaccessible objects.
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: The destination bucket must be in the same location as the CMEK. If the destination bucket is in another project, you must add the destination project's Cloud Storage service account to the key. 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: `generation`, `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.

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.

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.

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) and file size.

Metadata example	Preservation behavior
Modified time (mtime) and file size.	Preserved. mtime is preserved as custom metadata with the key `goog-reserved-file-mtime`. File size is preserved as `size`.
Folder permissions and hard links.	Not preserved. Storage Transfer Service does not create folder placeholder objects in Cloud Storage to represent folders.
Numeric UID, numeric GID, numeric MODE, and symbolic links.	Preservation behavior is specified with the `metadataOptions` object. See Preserving optional POSIX metadata for details. Default behavior is to not preserve any metadata.

Preserved.

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

Folder permissions and hard links.

Not preserved.

Storage Transfer Service does not create folder placeholder objects in Cloud Storage to represent folders.

Numeric UID, numeric GID, numeric MODE, and symbolic links.

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

Default behavior is to not preserve any metadata.

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.