The most efficient and Google-recommended method for migrating an on-premises Redis database to Memorystore for Redis is by using the native backup and import functionality. This process involves creating a standard Redis Database (RDB) snapshot file from the source instance, uploading this file to a Cloud Storage bucket, and then using the gcloud redis instances import command to load the data into the new Memorystore instance. This approach is optimized for bulk data transfer, minimizing migration time, cost, and the engineering effort required compared to developing a custom solution or using intermediate steps.

A. This adds an unnecessary intermediate migration to a Compute Engine instance, which increases complexity, cost, and overall migration time.

B. Writing a custom shell script is a high-effort, error-prone approach that is significantly less efficient than using the native RDB import feature.

C. Using Dataflow is overly complex and expensive for a direct database migration. It is designed for large-scale data processing, not a lift-and-shift database transfer.

1. Google Cloud Documentation - Memorystore for Redis, "Importing data into a Redis instance": This document explicitly outlines the recommended procedure. "To import an RDB file into a Memorystore for Redis instance, you first upload the RDB file to a Cloud Storage bucket, and then you initiate the import operation. The source RDB file can come from any Redis environment..." This confirms the steps in option D are the official, documented method.

Source: Google Cloud Documentation, "Importing data into a Redis instance", Overview section.

2. Google Cloud Documentation - Memorystore for Redis, "Migrating Redis to Memorystore for Redis": This guide details migration strategies. For offline migration, it states: "The simplest way to migrate your data is to use the import/export feature for Memorystore for Redis. You can export your data from your existing Redis instance to an RDB file, upload it to Cloud Storage, and then import it into your new Memorystore for Redis instance." This directly supports option D as the simplest, recommended practice.

Source: Google Cloud Documentation, "Migrating Redis to Memorystore for Redis", Offline migration section.

3. Google Cloud Documentation - gcloud redis instances import: The command-line reference for gcloud shows the specific tool used for this task. The command gcloud redis instances import SOURCE INSTANCE --region=REGION requires a SOURCE argument which is the "Google Cloud Storage location of the RDB file." This confirms the tooling and process described in option D.

Source: Google Cloud SDK Documentation, gcloud redis instances import.