Resilient File System (ReFS) overview

Applies to: Windows Server 2019, Windows Server 2016, Windows Server 2012 R2, Windows Server 2012, Windows Server (Semi-Annual Channel)

The Resilient File System (ReFS) is Microsoft’s newest file system, designed to maximize data availability, scale efficiently to large data sets across diverse workloads, and provide data integrity by means of resiliency to corruption. It seeks to address an expanding set of storage scenarios and establish a foundation for future innovations.

Key benefits

Resiliency

ReFS introduces new features that can precisely detect corruptions and also fix those corruptions while remaining online, helping provide increased integrity and availability for your data:

• Integrity-streams — ReFS uses checksums for metadata and optionally for file data, giving ReFS the ability to reliably detect corruptions.
• Storage Spaces integration — When used in conjunction with a mirror or parity space, ReFS can automatically repair detected corruptions using the alternate copy of the data provided by Storage Spaces. Repair processes are both localized to the area of corruption and performed online, requiring no volume downtime.
• Salvaging data — If a volume becomes corrupted and an alternate copy of the corrupted data doesn’t exist, ReFS removes the corrupt data from the namespace. ReFS keeps the volume online while it handles most non-correctable corruptions, but there are rare cases that require ReFS to take the volume offline.
• Proactive error correction — In addition to validating data before reads and writes, ReFS introduces a data integrity scanner, known as a scrubber. This scrubber periodically scans the volume, identifying latent corruptions and proactively triggering a repair of corrupt data.

Performance

In addition to providing resiliency improvements, ReFS introduces new features for performance-sensitive and virtualized workloads. Real-time tier optimization, block cloning, and sparse VDL are good examples of the evolving capabilities of ReFS, which are designed to support dynamic and diverse workloads:

Mirror-accelerated parity — Mirror-accelerated parity delivers both high performance and also capacity efficient storage for your data.

To deliver both high performance and capacity efficient storage, ReFS divides a volume into two logical storage groups, known as tiers. These tiers can have their own drive and resiliency types, allowing each tier to optimize for either performance or capacity. Some example configurations include:

Performance tier	Capacity tier
Mirrored SSD	Mirrored HDD
Mirrored SSD	Parity SSD
Mirrored SSD	Parity HDD

Once these tiers are configured, ReFS use them to deliver fast storage for hot data and capacity-efficient storage for cold data:

• All writes will occur in the performance tier, and large chunks of data that remain in the performance tier will be efficiently moved to the capacity tier in real-time.
• If using a hybrid deployment (mixing flash and HDD drives), the cache in Storage Spaces Direct helps accelerate reads, reducing the effect of data fragmentation characteristic of virtualized workloads. Otherwise, if using an all-flash deployment, reads also occur in the performance tier.

For Server deployments, mirror-accelerated parity is only supported on Storage Spaces Direct. We recommend using mirror-accelerated parity with archival and backup workloads only. For virtualized and other high performance random workloads, we recommend using three-way mirrors for better performance.

Accelerated VM operations — ReFS introduces new functionality specifically targeted to improve the performance of virtualized workloads:

• Block cloning — Block cloning accelerates copy operations, enabling quick, low-impact VM checkpoint merge operations.
• Sparse VDL — Sparse VDL allows ReFS to zero files rapidly, reducing the time needed to create fixed VHDs from 10s of minutes to mere seconds.

Variable cluster sizes — ReFS supports both 4K and 64K cluster sizes. 4K is the recommended cluster size for most deployments, but 64K clusters are appropriate for large, sequential IO workloads.

Scalability

ReFS is designed to support extremely large data sets—millions of terabytes—without negatively impacting performance, achieving greater scale than prior file systems.

Supported deployments

Microsoft has developed NTFS specifically for general-purpose use with a wide range of configurations and workloads, however for customers specially requiring the availability, resiliency, and/or scale that ReFS provides, Microsoft supports ReFS for use under the following configurations and scenarios.

All ReFS supported configurations must use Windows Server Catalog certified hardware and meet application requirements.

Storage Spaces Direct

Deploying ReFS on Storage Spaces Direct is recommended for virtualized workloads or network-attached storage:

• Mirror-accelerated parity and the cache in Storage Spaces Direct deliver high performance and capacity-efficient storage.
• The introduction of block clone and sparse VDL dramatically accelerates .vhdx file operations, such as creation, merge, and expansion.
• Integrity-streams, online repair, and alternate data copies enable ReFS and Storage Spaces Direct to jointly to detect and correct storage controller and storage media corruptions within both metadata and data.
• ReFS provides the functionality to scale and support large data sets.

Storage Spaces

• Integrity-streams, online repair, and alternate data copies enable ReFS and Storage Spaces to jointly to detect and correct storage controller and storage media corruptions within both metadata and data.
• Storage Spaces deployments can also utilize block-cloning and the scalability offered in ReFS.
• Deploying ReFS on Storage Spaces with shared SAS enclosures is suitable for hosting archival data and storing user documents.

Storage Spaces supports local non-removable direct-attached via BusTypes SATA, SAS, NVME, or attached via HBA (aka RAID controller in pass-through mode).

Basic disks

Deploying ReFS on basic disks is best suited for applications that implement their own software resiliency and availability solutions.

• Applications that introduce their own resiliency and availability software solutions can leverage integrity-streams, block-cloning, and the ability to scale and support large data sets.

If you plan to use ReFS for CSV (Cluster Shared Volumes), please consider the limitations to pre-format your later CSV volumes with ReFS. For CSV: NTFS should be used for traditional SANs. ReFS should be used on top of S2D.

Basic disks include local non-removable direct-attached via BusTypes SATA, SAS, NVME, or RAID. Basic disks do not include Storage Spaces.

Backup target

Deploying ReFS as a backup target is best suited for applications and hardware that implement their own resiliency and availability solutions.

• Applications that introduce their own resiliency and availability software solutions can leverage integrity-streams, block-cloning, and the ability to scale and support large data sets.

Backup targets include the above supported configurations. Please contact application and storage array vendors for support details on Fiber Channel and iSCSI SANs. For SANs, if features such as thin provisioning, TRIM/UNMAP, or Offloaded Data Transfer (ODX) are required, NTFS must be used.

NTFS overview

Applies to: Windows 10, Windows Server 2019, Windows Server 2016, Windows Server 2012 R2, Windows Server 2012, Windows Server 2008 R2, Windows Server 2008

NTFS—the primary file system for recent versions of Windows and Windows Server—provides a full set of features including security descriptors, encryption, disk quotas, and rich metadata, and can be used with Cluster Shared Volumes (CSV) to provide continuously available volumes that can be accessed simultaneously from multiple nodes of a failover cluster.

For additional feature information, see the Additional information section of this topic. To learn about the newer Resilient File System (ReFS), see Resilient File System (ReFS) overview.

Increased reliability

NTFS uses its log file and checkpoint information to restore the consistency of the file system when the computer is restarted after a system failure. After a bad-sector error, NTFS dynamically remaps the cluster that contains the bad sector, allocates a new cluster for the data, marks the original cluster as bad, and no longer uses the old cluster. For example, after a server crash, NTFS can recover data by replaying its log files.

NTFS continuously monitors and corrects transient corruption issues in the background without taking the volume offline (this feature is known as self-healing NTFS, introduced in Windows Server 2008). For larger corruption issues, the Chkdsk utility, in Windows Server 2012 and later, scans and analyzes the drive while the volume is online, limiting time offline to the time required to restore data consistency on the volume. When NTFS is used with Cluster Shared Volumes, no downtime is required. For more information, see NTFS Health and Chkdsk.

Increased security

Access Control List (ACL)-based security for files and folders—NTFS allows you to set permissions on a file or folder, specify the groups and users whose access you want to restrict or allow, and select access type.

Support for BitLocker Drive Encryption—BitLocker Drive Encryption provides additional security for critical system information and other data stored on NTFS volumes. Beginning in Windows Server 2012 R2 and Windows 8.1, BitLocker provides support for device encryption on x86 and x64-based computers with a Trusted Platform Module (TPM) that supports connected stand-by (previously available only on Windows RT devices). Device encryption helps protect data on Windows-based computers, and it helps block malicious users from accessing the system files they rely on to discover the user’s password, or from accessing a drive by physically removing it from the PC and installing it on a different one. For more information, see What’s new in BitLocker.

Support for large volumes

NTFS can support volumes as large as 8 petabytes on Windows Server 2019 and newer and Windows 10, version 1709 and newer (older versions support up to 256 TB). Supported volume sizes are affected by the cluster size and the number of clusters. With (2 32 – 1) clusters (the maximum number of clusters that NTFS supports), the following volume and file sizes are supported.

Cluster size	Largest volume and file
4 KB (default size)	16 TB
8 KB	32 TB
16 KB	64 TB
32 KB	128 TB
64 KB (earlier max)	256 TB
128 KB	512 TB
256 KB	1 PB
512 KB	2 PB
1024 KB	4 PB
2048 KB (max size)	8 PB

Note that if you try to mount a volume with a cluster size larger than the supported maximum of the version of Windows you’re using, you get the error STATUS_UNRECOGNIZED_VOLUME.

Services and apps might impose additional limits on file and volume sizes. For example, the volume size limit is 64 TB if you’re using the Previous Versions feature or a backup app that makes use of Volume Shadow Copy Service (VSS) snapshots (and you’re not using a SAN or RAID enclosure). However, you might need to use smaller volume sizes depending on your workload and the performance of your storage.

Formatting requirements for large files

To allow proper extension of large .vhdx files, there are new recommendations for formatting volumes. When formatting volumes that will be used with Data Deduplication or will host very large files, such as .vhdx files larger than 1 TB, use the Format-Volume cmdlet in Windows PowerShell with the following parameters.

Parameter	Description
-AllocationUnitSize 64KB	Sets a 64 KB NTFS allocation unit size.
-UseLargeFRS	Enables support for large file record segments (FRS). This is needed to increase the number of extents allowed per file on the volume. For large FRS records, the limit increases from about 1.5 million extents to about 6 million extents.

For example, the following cmdlet formats drive D as an NTFS volume, with FRS enabled and an allocation unit size of 64 KB.

You also can use the format command. At a system command prompt, enter the following command, where /L formats a large FRS volume and /A:64k sets a 64 KB allocation unit size:

Maximum file name and path

NTFS supports long file names and extended-length paths, with the following maximum values:

Support for long file names, with backward compatibility—NTFS allows long file names, storing an 8.3 alias on disk (in Unicode) to provide compatibility with file systems that impose an 8.3 limit on file names and extensions. If needed (for performance reasons), you can selectively disable 8.3 aliasing on individual NTFS volumes in Windows Server 2008 R2, Windows 8, and more recent versions of the Windows operating system. In Windows Server 2008 R2 and later systems, short names are disabled by default when a volume is formatted using the operating system. For application compatibility, short names still are enabled on the system volume.

Support for extended-length paths—Many Windows API functions have Unicode versions that allow an extended-length path of approximately 32,767 characters—beyond the 260-character path limit defined by the MAX_PATH setting. For detailed file name and path format requirements, and guidance for implementing extended-length paths, see Naming Files, Paths, and Namespaces.

Clustered storage—When used in failover clusters, NTFS supports continuously available volumes that can be accessed by multiple cluster nodes simultaneously when used in conjunction with the Cluster Shared Volumes (CSV) file system. For more information, see Use Cluster Shared Volumes in a Failover Cluster.

Flexible allocation of capacity

If the space on a volume is limited, NTFS provides the following ways to work with the storage capacity of a server:

• Use disk quotas to track and control disk space usage on NTFS volumes for individual users.
• Use file system compression to maximize the amount of data that can be stored.
• Increase the size of an NTFS volume by adding unallocated space from the same disk or from a different disk.
• Mount a volume at any empty folder on a local NTFS volume if you run out of drive letters or need to create additional space that is accessible from an existing folder.