GCS Data Transfer Operations

This reference guide provides detailed guidelines, a decision tree, and commands for transferring data in and out of Google Cloud Storage (GCS). It covers single file transfers, handling large files, managing high-concurrency transfers of many small files, parallelizing using gcloud storage, and setting up Storage Transfer Service (STS).

Decision Tree: Choosing the Best Transfer Method

Use the following flowchart and logic to choose the most efficient data transfer method based on your host environment, dataset size, file count, and GCS bucket settings.

graph TD
    Start([Start: Identify Transfer Requirements]) --> SourceCheck{Source Location?}

    SourceCheck -- "AWS S3 / Azure" --> STS[Storage Transfer Service - Agentless]

    SourceCheck -- "On-Premises Filesystem" --> OnPremSize{Data Volume?}
    OnPremSize -- "< 1 TB" --> SecCp[gcloud storage cp / rsync]
    OnPremSize -- ">= 1 TB" --> STSagent[Storage Transfer Service - Agent-based]

    SourceCheck -- "Local / GCS-to-GCS" --> VolumeCheck{Object Count?}

    VolumeCheck -- "Few Files (<10K objects)" --> SizeCheck{File Size?}
    SizeCheck -- "< 150 MB" --> SecCp
    SizeCheck -- ">= 150 MB" --> LargeConfig[gcloud storage cp with PCU and Slices]

    VolumeCheck -- "Many Files (>=10K objects)" --> ParallelCp[gcloud storage cp / rsync folder]

Detailed Decision Guidelines:

1. Cross-Cloud Migration (AWS S3, Azure Blob Storage):
   • Recommendation: Always use Storage Transfer Service (STS). It runs serverless on Google infrastructure for cloud-to-cloud transfers, avoiding the need to funnel petabytes of data through a local machine or VM, saving egress and compute costs.
2. On-Premises Migration:
   • Recommendation: If the dataset is under 1TB, it is generally simpler to use the gcloud storage CLI (e.g. gcloud storage cp -r or rsync). If the dataset is 1TB or larger, configure Storage Transfer Service (STS) using on-premises transfer agents.
3. Few Large Files (size >= 150MB, count < 10K):
   • Recommendation: Use gcloud storage cp. By default, it enables Parallel Composite Uploads (for writing) and Sliced Downloads (for reading) to thread chunk transfers.

Inspecting Bucket Configuration

Before selecting a metadata transfer or loading strategy, query your bucket's features to check for conflicts (such as Soft Delete, Versioning, Retention/Lock policies, or storage class):

gcloud storage buckets describe gs://my-bucket/ --format="json(softDeletePolicy,versioning,retentionPolicy,storageClass)"

1. Single File Upload and Download with gcloud CLI

For simple, single-file transfers, use the gcloud storage cp command.

• Upload a local file to GCS:

  gcloud storage cp /path/to/local/file.txt gs://my-bucket/destination-folder/
  

• Download an object from GCS to local directory:

  gcloud storage cp gs://my-bucket/destination-folder/file.txt /path/to/local/dir/
  

2. Transferring Large Files (Parallel Upload & Sliced Download)

When dealing with large files, the gcloud CLI automatically optimizes performance by splitting files into chunks for concurrent uploads and downloads.

A. Parallel Composite Uploads (PCU)

By default, gcloud storage cp uses GCS JSON API's Parallel Composite Uploads for large files.

• How it works: The source file is divided into chunks, which are uploaded in parallel to GCS as temporary composite components. The components are then merged into a single final GCS object using the GCS compose API.

• Checksum behavior: Because it uses the compose API, the final assembled object will have a CRC32C checksum instead of an MD5 hash. Non-composite uploads — single-shot or resumable, which is what gcloud storage cp performs when the file is below the threshold or PCU is disabled — do store an MD5 hash, so pipelines that verify per-object MD5 should disable PCU (or raise the threshold) rather than work around it.

• Tuning Properties:

  • Enable/Disable:

    gcloud config set storage/parallel_composite_upload_enabled True  # Or False to disable
    

  • Threshold:

    gcloud config set storage/parallel_composite_upload_threshold 150M  # e.g., 150MB
    

  • Component Size:

    gcloud config set storage/parallel_composite_upload_component_size 50M # e.g., 50MB
    

[!IMPORTANT] Do not use Parallel Composite Uploads if your bucket has a retention policy enabled, because the temporary objects can't be deleted until they meet the retention period. By default, gcloud protects against this: storage/parallel_composite_upload_compatibility_check (default True) skips PCU unless the object's storage class is STANDARD and the destination bucket has no retention policy and no default object holds — the conflict only arises if that check has been set to False. While Soft Delete and Object Versioning do not prevent PCU from working, they can significantly increase storage costs because deleted temporary components remain on the bucket.

For pipelines that compose objects themselves (chunked uploads merged with gcloud storage objects compose), avoid those soft-delete/versioning costs by hard-deleting the temporary components during composition instead of deleting them afterwards: gcloud storage objects compose gs://B/chunk1 gs://B/chunk2 gs://B/final --delete-source-objects hard-deletes the source objects as part of the compose operation, so they never become soft-deleted or noncurrent versions while bucket-level protections stay on for the final objects.

B. Sliced Object Downloads

For downloading large files, the gcloud CLI executes ranged GET requests (slices) in parallel, writing files concurrently into pre-allocated placeholders in the local destination.

• Checksum validation: To verify the data integrity of the downloaded slices, gcloud uses CRC32C checksums.

• Tuning Properties:

  • Threshold:

    gcloud config set storage/sliced_object_download_threshold 0  # Set to 0 to disable sliced downloads
    

[!IMPORTANT] Sliced downloads require a compiled crcmod Python package on the host machine. If a compiled crcmod is not available, gcloud CLI prints a warning and automatically falls back to slower sequential downloading. By default, gcloud dynamically autotunes slice triggers and components.

4. Scaling Concurrent Multi-File Transfers with gcloud CLI

gcloud storage is natively thread-safe and parallel by default.

Tuning Concurrency Controls

Concurrency inside gcloud is auto-tuned by default. If performance is poor or resource limits are hit, process and thread values can be adjusted:

1. Process Count:

   gcloud config set storage/process_count 8
   # Or env variable: CLOUDSDK_STORAGE_PROCESS_COUNT=8
   

2. Thread Count:

   gcloud config set storage/thread_count 8
   # Or env variable: CLOUDSDK_STORAGE_THREAD_COUNT=8
   

To disable concurrency entirely for sequential execution (e.g. debugging, rate limit issues), set both values to 1:

gcloud config set storage/process_count 1
gcloud config set storage/thread_count 1

To recursively upload or download directories containing many files, use the --recursive or -r flag:

# Recursively upload a local directory to a bucket in parallel
gcloud storage cp -r ./local-directory gs://my-bucket/

# Recursively download a bucket prefix to a local directory in parallel
gcloud storage cp -r gs://my-bucket/backup-dir/ ./local-restore-dir/

Syncing Directories with gcloud storage rsync

For synchronizing rather than copying, use rsync, which also runs in parallel by default:

# Sync local directory to cloud (compares modification time and size, uploads missing/modified files, doesn't delete unmatched)
gcloud storage rsync ./my-data-folder gs://my-bucket/synced-data/ --recursive

5. Storage Transfer Service (STS) for S3 and Azure Migrations

For multi-terabyte or petabyte-scale data transfers from Amazon S3, Microsoft Azure, or public/private HTTP sources to GCS, use the Storage Transfer Service (STS). STS runs transitions between cloud providers asynchronously on Google infrastructure without using local network or VM bandwidth. (Transfers from on-premises sources require deploying on-premises transfer agents).

STS also supports Amazon S3 sources in AWS GovCloud (US) regions (us-gov-east-1 and us-gov-west-1), for both one-time/batch and event-driven transfers. GovCloud sources use the arn:aws-us-gov: ARN prefix (for example arn:aws-us-gov:iam::AWS_ACCOUNT:role/ROLE_NAME); transfers over the managed private network are not available for GovCloud regions.

A. Prerequisites for Storage Transfer Service (STS)

Before creating a transfer job, complete the following steps:

1. Enable the API:

   gcloud services enable storagetransfer.googleapis.com
   

2. Grant the STS service agent access to the destination bucket: The service agent (project-PROJECT_NUMBER@storage-transfer-service.iam.gserviceaccount.com) needs roles/storage.legacyBucketWriter on the destination bucket. If the overwrite option is set to check for "different" or "never", it also needs roles/storage.objectViewer.

   • Grant permission command:

     gcloud storage buckets add-iam-policy-binding gs://DESTINATION_BUCKET_NAME \
         --member="serviceAccount:project-PROJECT_NUMBER@storage-transfer-service.iam.gserviceaccount.com" \
         --role="roles/storage.legacyBucketWriter"
     

B. Credentials JSON File Setup

To authorize read access to your source without putting secret keys into CLI command arguments, construct a local JSON credential file.

• For Amazon S3 (s3-creds.json):

  {
    "accessKeyId": "AWS_ACCESS_KEY_ID",
    "secretAccessKey": "AWS_SECRET_ACCESS_KEY"
  }
  

• For Azure Blob Storage (azure-creds.json):

  {
    "sasToken": "AZURE_SHARE_ACCESS_SIGNATURE_TOKEN"
  }
  

C. STS Commands

Create a Transfer Job from Amazon S3:

gcloud transfer jobs create s3://my-aws-s3-bucket/ gs://my-gcs-destination-bucket/ \
    --source-creds-file=s3-creds.json \
    --description="One-time S3 migration to GCS" \
    --name="s3-to-gcs-migration"

Create a Transfer Job from Azure Blob Storage:

Specify the Azure Blob Storage container endpoint as the source:

gcloud transfer jobs create https://myazurestorageaccount.blob.core.windows.net/mycontainer/ gs://my-gcs-destination-bucket/ \
    --source-creds-file=azure-creds.json \
    --description="One-time Azure container migration to GCS" \
    --name="azure-to-gcs-migration"

Additional STS CLI Operations:

• Describe the job status and details:

  gcloud transfer jobs describe s3-to-gcs-migration
  

• Run a saved transfer job immediately:

  gcloud transfer jobs run s3-to-gcs-migration
  

• Monitor progress of the current transfer run in real time:

  gcloud transfer jobs monitor s3-to-gcs-migration
  

• List all existing STS transfer jobs:

  gcloud transfer jobs list