Dev Drive in Windows 11: Faster Builds with ReFS and Defender Performance Mode

Windows 11’s Dev Drive is one of those under-the-radar features that can materially shorten build times, speed up repository operations, and reduce the friction of daily developer work—provided you use it the way Microsoft intended. In short: Dev Drive is a ReFS-formatted storage volume tuned for developer workloads, combined with a Defender “performance mode” that defers synchronous scanning for files on that volume. The result is measurable and repeatable improvement on many common tasks (git clones, package restores, builds and large-file copy operations), plus a management model that gives you control over what gets scanned and what doesn’t. Evidence from Microsoft’s documentation and engineering posts shows single-digit-to-double-digit percent wins across many repos, and much larger wins in copy-heavy scenarios; community tests echo those gains while also showing variability depending on hardware and workload.

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Background / Overview​

Dev Drive is a new, purpose-built storage volume for developer workloads in Windows 11. It is:

• Built on the Resilient File System (ReFS) but tuned for developer scenarios rather than general purpose storage.
• Designed to host source code repositories, package caches, build artifacts, temporary and intermediate files—not the OS or large installed apps like Visual Studio itself.
• Integrated with a Defender capability called Performance Mode, which allows asynchronous (deferred) scans for files on a trusted Dev Drive, improving file-open and write latency for developer operations.

Microsoft first published Dev Drive guidance and blog posts as it rolled the feature through Insider channels and into stable releases. Those posts show optimization work across the stack—filesystem metadata, copy-on-write features, and Defender integration—targeted at the developer inner loop (clone → build → test → iterate). Independent community experiments and a handful of vendor/engineering blogs confirm noticeable speedups in real-world scenarios, while also warning that results depend heavily on workload shape, disk hardware, and configuration choices.

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Why Dev Drive matters: three core advantages​

1) It’s faster for developer IO patterns​

Dev Drive’s ReFS tuning and Defender performance mode primarily accelerate operations that are common in development workflows:

• Repository operations (clones, checkouts, clean-ups) that create or remove many small files.
• Package caches read/writes (npm, NuGet, Maven).
• Build output and intermediate files that are created and deleted repeatedly during incremental builds.
• Large file copy and snapshot operations where block cloning (a form of copy-on-write) can make copies near-instantaneous.

Microsoft’s documentation and engineering posts show a range of improvements: early marketing noted “up to ~30% faster” build times in certain scenarios when moving from NTFS to Dev Drive and enabling performance mode, while internal engineering benchmarking showed wins that vary by repo and workload, sometimes hitting the low 40s in percentage improvement for specific repos under specific conditions. For very large file copies, newer b-cloning engine work demonstrated dramatic reductions (e.g., multi-second operations reduced to sub-second durations in Microsoft’s published examples). Those figures are real results for their test hardware and workloads — you should expect variation in practice.
Why is Dev Drive faster? A short technical summary:

• ReFS avoids some NTFS metadata bottlenecks and includes optimizations for directory and metadata operations that matter when a build system churns lots of files.
• Defender’s performance mode turns synchronous scanning into asynchronous scanning for trusted Dev Drives—files are opened and the file operation completed quickly, with malware scans deferred to run after the operation, reducing IO wait.
• Copy-on-write / block cloning reduces the work and storage load when files are duplicated, critical for package managers and certain build tasks that copy large caches or blobs. Microsoft has continued investing in block cloning for near-instant copies on Dev Drives.

2) It’s secure when used correctly​

At first glance, “tell Defender to scan less” may sound dangerous. Microsoft implemented a model that balances developer performance and system security:

• A Dev Drive must be designated as trusted to enable Defender performance mode. That trust designation is explicit and reversible. The Windows docs describe the commands and Settings UI to create and inspect trust status.
• Performance Mode does not disable Defender altogether; it changes the scanning model (asynchronous scanning for file-open operations on the trusted volume) to reduce blocking latency without leaving files entirely unscanned.
• Microsoft recommends the Dev Drive host only files you trust: source repos, package caches, and build outputs. It explicitly warns against storing random downloads or general-purpose applications there. The system drive (C cannot be a Dev Drive, and removable drives are not supported as trusted Dev Drive volumes.

When you follow Microsoft’s guidance—keeping the Dev Drive limited to developer-controlled artifacts and not using it as a general downloads or application install location—the attack surface remains manageable. That said, the model requires developer discipline: a Dev Drive is a tool for trusted code and should be treated accordingly.

3) It’s flexible: partition or VHD, fixed or dynamic​

Dev Drives can be created either as a classic physical partition formatted for Dev Drive, or as a VHD/VHDX file that acts as a virtual disk. Each approach has trade-offs:

• Physical partition: slightly better raw performance because there’s no virtual disk layer. Less flexible; the drive is tied to the machine.
• VHD/VHDX (virtual disk): a file on your host volume that mounts as a Dev Drive. Dynamic resizing is supported (dynamically expanding VHDs) and the VHD file can be copied or backed up easily. Microsoft cautions against moving a Dev Drive VHD between machines and continuing to use it as a trusted Dev Drive, but the portability for transport/backup is a useful convenience.

Additional practical notes: minimum Dev Drive size is 50 GB, Microsoft recommends 16 GB RAM (8 GB minimum), and there are limitations—Dev Drives cannot be the system drive and dynamic disks are unsupported for Dev Drive usage. BitLocker will encrypt a VHD if its host volume is BitLocker-protected.

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Deep dive: how performance mode and ReFS combine​

ReFS tuning for developer workloads​

ReFS was designed for data integrity and scale, but for Dev Drive Microsoft has retuned and optimized ReFS for developer patterns: metadata handling, directory operations, and fast allocation patterns that benefit builds and package caches. Those changes are lower-level but result in observable improvements in workloads where tens of thousands of small file operations are the norm. Engineering write-ups from Microsoft describe measured wins when moving large codebases to a Dev Drive, with compounded benefits when build systems are modified to use copy-on-write-friendly operations.

Defender Performance Mode: asynchronous scanning​

Conventional real-time protection scans files synchronously during open/read/write. That means the process performing the file IO waits for the scan to complete—introducing latency. Dev Drive’s Performance Mode switches Defender’s behavior for trusted Dev Drives so that the file open returns quickly and the scan proceeds in the background. This reduces blocking latency for builds and tooling that open many files rapidly. Microsoft documents the requirements for performance mode (antimalware platform version, Defender settings, and when performance mode is enabled). Administrators can also enforce the setting via Intune or Group Policy for managed machines.

Copy-on-Write and block cloning: immediate copies for big files​

A separate but related optimization is block cloning (copy-on-write semantics inside the Windows copy engine) that drastically reduces the cost to duplicate files. Instead of copying every block, block cloning makes a metadata-level link and populates new blocks only when a write occurs. Microsoft’s published examples show copy time reductions for large files measured in the tens-to-hundreds of milliseconds versus multiple seconds for NTFS copies—an enormous win when your workflow frequently duplicates large images, blobs, or package caches. These features continue to evolve and are part of the reason Dev Drive’s performance gains have improved with successive Windows releases.

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Practical setup: quick checklist and steps​

Before you create a Dev Drive, verify these prerequisites:

• Windows 11 build that includes Dev Drive support (check Windows Update / Microsoft Docs).
• Minimum 50 GB free for the Dev Drive.
• Recommended 16 GB RAM; 8 GB minimum.
• Microsoft Defender enabled and up to date for Defender Performance Mode to operate.

Simple creation paths:

• Settings → System → Storage → Advanced storage settings → Disks & volumes → Create Dev Drive. Follow the wizard to allocate either free space or to create a VHDX.
• Or run command-line formatting: Format D: /DevDrv /Q or PowerShell Format-Volume -DriveLetter D -DevDrive (requires Admin).
• Once created, designate the Dev Drive as trusted (fsutil devdrv trust <drive-letter>), or query trust with fsutil devdrv query <drive-letter>.

Short recommended setup pattern:

• Keep developer tools (Visual Studio, SDKs) on C:, move repositories, package caches and build outputs to the Dev Drive. This follows Microsoft guidance and avoids unexpected tool behavior.
• Use dynamic VHDX for portability and efficient disk usage if you plan to move or back up the Dev Drive file (but avoid using a moved VHD as a trusted Dev Drive on another machine without re-creating trust).
• Enable BitLocker on the host volume (if you need encryption); mounted VHDs inherit BitLocker protection from the hosting volume.

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Real-world results: what you’ll likely see (and what you won’t)​

Multiple Microsoft studies and community tests show meaningful reductions in build and repository operation times, but the exact percentage varies:

• Microsoft’s Windows Developer Blog and engineering posts document scenarios from ~14% faster full builds up to 28% when combining Dev Drive with copy-on-write build optimizations; specific repos occasionally show higher wins in controlled testing. For one git task (cloning a large repo), community recounts and reporting include results like 41% faster clone times in some configurations—an eye-catching number but not universal. Expect a range: small gains for CPU-bound builds, larger gains for IO-bound repos and workflows.
• For large file copy operations, b-cloning improvements reported by Microsoft show orders-of-magnitude reductions (e.g., multi-second copies down to sub-second). If your workload regularly duplicates large binary blobs, the impact can be dramatic.

What you should not expect:

• Universal 2x speedups. The gains depend on whether the workload is IO-limited or CPU-limited. If your build is dominated by compilation CPU time, faster IO helps less.
• That Dev Drive replaces general security hygiene. It’s a targeted tool; storing untrusted downloads on a Dev Drive undermines its security model and exposes you to risk.

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Risk assessment and caveats​

Dev Drive is a great tool when used as intended, but it carries specific trade-offs and potential pitfalls.

• Security discipline required: Trusting a drive changes Defender’s scanning behavior for that volume. Only put trusted, developer-controlled artifacts there. Do not use a Dev Drive as a sandbox for arbitrary downloads.
• Variability in performance: Microsoft’s own engineering posts caution that not every repository sees the same uplift; some saw 10% or less, others 40%+. Real-world variability comes from hardware, toolchain, parallelism, and repo layout. Plan a short trial on your own workload before making Dev Drive central to your workflow.
• Portability limits for VHDs: Microsoft recommends against copying a VHD-hosted Dev Drive to another machine and continuing to use it as a Dev Drive without re-creating trust or reformatting. The portability is useful for transport and backups, but cross-machine reuse as a trusted volume is discouraged.
• Compatibility and enterprise policy: In managed environments, IT policies may restrict trust designation and Defender configuration; consult your admin before deploying Dev Drives widely in an enterprise.
• ReFS resource usage: ReFS can use slightly more memory than NTFS; Microsoft recommends adequate RAM for best results. On constrained machines you may see different trade-offs.

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Best practices: recommended workflow for developers​

• Use Dev Drive for:
• Source code repositories (git).
• Package caches (npm, NuGet, Maven).
• Build outputs, intermediate files, and temp folders.
• Do not use Dev Drive for:
• Operating system or core applications (the C: system drive cannot be a Dev Drive anyway).
• Random downloads, web browser downloads, or unknown binaries.
• Persistent installs of Visual Studio or other large IDEs.
• Practical guidelines:
• Benchmark your own repository: measure baseline clone, restore and build times on your current drive, create a Dev Drive, move caches and sources, and re-run tests to get real data.
• Keep tooling on C:, but point package managers and build outputs to the Dev Drive (Visual Studio recognizes Dev Drive as a valid project location).
• Use dynamic VHDX for flexible storage if you need backups or to copy the drive file—just avoid continued trusted reuse on other hardware without re-creating the Dev Drive properly.
• Use BitLocker on the host if you need encryption; VHDs inherit host encryption.

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How to evaluate Dev Drive for your team in 6 steps​

• Identify an IO-bound project with significant file churn (large repo, lots of package restores).
• Record baseline timings for clone, restore and full build on your current storage.
• Create a Dev Drive (VHDX if you want safety and flexibility), move caches and repo, and mark it trusted per docs.
• Re-run the same tasks and measure differences.
• If gains are meaningful, codify a team policy: what can go on the Dev Drive, how to back it up, and how to handle security.
• Monitor for regressions after Windows updates; Microsoft and the community continue to iterate on Dev Drive and ReFS, so re-test periodically.

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Final analysis: who should adopt Dev Drive and when​

Dev Drive is a focused optimization for developers and teams that meet these conditions:

• Your workflows are IO-bound (lots of file creation/deletion, large caches, frequent copies).
• You can enforce discipline about what goes onto the Dev Drive (trusted sources only).
• You have hardware and memory to support ReFS overhead (Microsoft recommends 8–16 GB of RAM).

If your builds are dominated by CPU compilation time or remote CI does the heavy lifting, Dev Drive may provide modest improvements. If your day-to-day development involves large local monorepos, package managers with huge caches, or repeated duplication of large binaries, the gains can be substantial—and sometimes dramatic for copy-heavy tasks thanks to block cloning. Microsoft’s documentation and engineering posts, alongside community testing, give a consistent picture: Dev Drive is not a panacea, but it is a pragmatic, well-supported optimization that can reclaim developer time when used correctly.

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Dev Drive is a modern example of narrow, pragmatic engineering: targeted filesystem improvements plus a security-aware scanning model that together reduce friction for developers. Used carefully—by placing only trusted project artifacts on the volume, following Microsoft’s guidance for trust and Defender configuration, and verifying gains on your own workloads—Dev Drive can be a very effective addition to your Windows install. For many developers the ROI is immediate: shorter inner-loop cycles, faster local developer tasks, and fewer minutes wasted waiting for clones and builds to finish. If that sounds like the kind of daily productivity boost you need, a short pilot to measure real-world gains is the recommended next step.

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Source: xda-developers.com 3 reasons a Dev Drive might be the perfect addition to your Windows install