A company is migrating a legacy application that relies on the NFS protocol for shared data access across multiple Linux servers. The data is accessed frequently and is mission-critical. The best Google Cloud storage service that provides a fully managed, scalable, and highly available equivalent to this on-premises file storage is: Cloud Filestore (Enterprise or High Scale) Cloud Bigtable Cloud Storage (Standard) Compute Engine Persistent Disk (Multi-Attach)

The question requires a fully managed, scalable, and highly available storage service that supports the NFS protocol for shared access, which is a direct replacement for a legacy on-premises file storage system. Cloud Filestore is Google Cloud's fully managed Network File System (NFS) service, designed specifically for this use case. It allows multiple Linux servers (Compute Engine VMs) to concurrently access a shared file system. The Enterprise and High Scale tiers provide the high availability and performance required for mission-critical applications. Other options are unsuitable: Cloud Storage is object storage, Bigtable is a NoSQL database, and Persistent Disk provides block storage, none of which natively serve the NFS protocol as a managed service.

1. Google Cloud Documentation, "Filestore overview". The first paragraph explicitly states, "Filestore is a fully managed, high-performance file storage service from Google Cloud for applications that require a file system interface and a shared file system for data. Filestore provides a familiar file system experience and is ideal for enterprise applications that depend on NFS."

Source: https://cloud.google.com/filestore/docs/overview

2. Google Cloud Architecture Center, "File storage on Compute Engine". This document compares storage options and recommends Filestore for specific scenarios. In the "Choosing a file storage solution" section, it states, "For workloads that require a Network File System (NFS) protocol and shared file access, Filestore is an effective storage option."

Source: https://cloud.google.com/architecture/file-storage-on-compute-engine

3. Google Cloud Documentation, "Cloud Storage products". This page provides a comparison table of Google's storage services. It categorizes Filestore as "File storage" with the primary access method being "NFSv3" and the use case as "Managed network file share." This clearly distinguishes it from Persistent Disk (Block storage), Cloud Storage (Object storage), and Cloud Bigtable (NoSQL database).

Source: https://cloud.google.com/storage/docs/products