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.

Note

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.

Note

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.

Note

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.

Note

Basic disks include local non-removable direct-attached via BusTypes SATA, SAS, NVME, or RAID.

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.

Note

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.

Feature comparison

Limits

Feature ReFS NTFS
Maximum file name length 255 Unicode characters 255 Unicode characters
Maximum path name length 32K Unicode characters 32K Unicode characters
Maximum file size 35 PB (petabytes) 256 TB
Maximum volume size 35 PB 256 TB

Functionality

The following features are available on ReFS and NTFS:

Functionality ReFS NTFS
BitLocker encryption Yes Yes
Data Deduplication Yes1 Yes
Cluster Shared Volume (CSV) support Yes Yes
Soft links Yes Yes
Failover cluster support Yes Yes
Access-control lists Yes Yes
USN journal Yes Yes
Changes notifications Yes Yes
Junction points Yes Yes
Mount points Yes Yes
Reparse points Yes Yes
Volume snapshots Yes Yes
File IDs Yes Yes
Oplocks Yes Yes
Sparse files Yes Yes
Named streams Yes Yes
Thin Provisioning Yes2 Yes
Offloaded Data Transfer (ODX) No Yes
Trim/Unmap Yes2 Yes
  1. Available on Windows Server, version 1709 and later.
  2. Storage Spaces only

The following features are only available on ReFS:

Functionality ReFS NTFS
Block clone Yes No
Sparse VDL Yes No
Mirror-accelerated parity Yes (on Storage Spaces Direct) No

The following features are unavailable on ReFS at this time:

Functionality ReFS NTFS
File system compression No Yes
File system encryption No Yes
Transactions No Yes
Hard links No Yes
Object IDs No Yes
Short names No Yes
Extended attributes No Yes
Disk quotas No Yes
Bootable No Yes
Page file support No Yes
Supported on removable media No Yes

See also