Resilient File System (ReFS), codenamed "Protogon", is a Microsoft proprietary file system introduced with Windows Server 2012 with the intent of becoming the "next generation" file system after NTFS.
ReFS was designed to overcome issues that had become significant over the years since NTFS was conceived, related to how data storage requirements had changed. Its key design advantages are intended to include - automatic integrity checking and data scrubbing, removing the need for chkdsk and protecting against bit rot; built-in handling of hard drive failure and redundancy, including RAID and a switch to copy/allocate on write for data and metadata updates; very long path and filename handling; and storage virtualization and pooling, including almost arbitrary logical volume size (as distinct from the physical sizes of the disks used).
These requirements arose from two major changes in storage systems and usage - the size of storage in use (large or massive arrays of multi-terabyte drives now being fairly common), and the need for continual reliability. As a result the file system needs to be self repairing (to prevent disk checking from being impractically slow or disruptive), along with abstraction or virtualization between physical disks and logical volumes.
ReFS was initially added to Windows Server 2012 only, with the aim of gradual migration to consumer systems in future versions (although modifications were quickly developed by enthusiasts for the latter). The initial versions removed some NTFS features, causing concern among onlookers, such as quota systems and extended attributes. Some of these were re-implemented in later versions of ReFS.
In its early versions (2012 - 2013), ReFS was similar or slightly faster than NTFS in most tests, but far slower when full integrity checking was enabled, a result attributed to the relative newness of ReFS. Concerns were also raised over storage spaces, the storage system designed to underpin ReFS, which is able to fail in a manner that prevents ReFS itself from recovering.
- Improved reliability for on-disk structures
- ReFS uses B+ trees for all on-disk structures including metadata and file data. Metadata and file data are organized into tables similar to a relational database. The file size, number of files in a folder, total volume size and number of folders in a volume are limited by 64-bit numbers; as a result ReFS supports a maximum file size of 16 Exabytes, a maximum of 18.4 × 1018 folders and a maximum volume size of 1 Yottabyte (with 64 KB clusters) which allows large scalability with no practical limits on file and folder size (hardware restrictions still apply). Free space is counted by a hierarchical allocator which includes three separate tables for large, medium, and small chunks.
- Built-in resilience
- ReFS employs an allocation-on-write update strategy for metadata, which allocates new chunks for every update transaction and uses large IO batches. All ReFS metadata has built-in 64-bit checksums which are stored independently. The file data can have an optional checksum in a separate "integrity stream", in which case the file update strategy also implements allocation-on-write; this is controlled by a new "integrity" attribute applicable to both files and directories. If nevertheless file data or metadata becomes corrupt, the file can be deleted without taking down the whole volume offline for maintenance, then restored from the backup. As a result of built-in resiliency, administrators do not need to periodically run error-checking tools such as CHKDSK when using ReFS.
- Compatibility with existing APIs and technologies
- ReFS supports only a subset of NTFS features, and only Win32 APIs that are "widely adopted"; but does not require new system APIs and most file system filters continue to work with ReFS volumes. ReFS supports many existing Windows and NTFS features such as BitLocker encryption, Access Control Lists, USN Journal, change notifications, symbolic links, junction points, mount points, reparse points, volume snapshots, file IDs, and oplock. ReFS seamlessly integrates with Storage Spaces, a storage virtualization layer that allows data mirroring and striping, as well as sharing storage pools between machines. ReFS resiliency features enhance the mirroring feature provided by Storage Spaces and can detect whether any mirrored copies of files become corrupt using background data scrubbing process, which periodically reads all mirror copies and verifies their checksums then replaces bad copies with good ones.
Some NTFS features were removed and unsupported in the initial versions of ReFS. These included named streams, object IDs, 8.3 filename, NTFS compression, Encrypting File System (EFS), transactional NTFS, hard links, extended attributes, and disk quotas. ReFS does not itself offer data deduplication. In addition, Windows cannot be booted from a ReFS volume. Dynamic disks with mirrored or striped volumes are replaced with mirrored or striped storage pools provided by Storage Spaces, however, automated error-correction is only supported on mirrored spaces.
Features initially removed include:
- File-based compression
- Disk quotas
- Object identifiers
- Encrypting File System
- Named stream
- Hard links
- Extended file attributes
Windows 8.1 is the first client operating system to provide some support for ReFS.
ReFS was initially unsuitable for Microsoft SQL Server instance allocation due to the absence of alternate data streams. However, in Windows 8.1 and Server 2012 R2, ReFS reacquired alternate data streams and automatic correction of corruption when integrity streams are used on parity spaces.
Stability and known issues
Adding thin-provisioned ReFS on top of Storage Spaces (according to a 2012 pre-release article) can fail in a non-graceful manner, in which the volume without warning becomes inaccessible or unmanageable. This can happen, for example, if the physical disks underlying a storage space becomes too full. Smallnetbuilder comments that in such cases, recovery could be "prohibitive" as a "breakthrough in theory" is needed to identify storage space layouts and recover them, which is required before any ReFS recovery of file system contents can be started; therefore it recommends using backups as well.
Other issues identified or suggested for ReFS running on Storage Spaces (its intended design) include:
- Because Storage Spaces operates on blocks and not files, there is a higher risk of issues, since some files potentially cannot be read without the entire storage space working correctly. As a result, disk and data addition and removal may be impaired, and redundancy conversion becomes difficult or impossible.
- The system does not re-balance storage use, and does not make full use of disks of differing sizes, therefore only hard drives of identical size can be efficiently combined.
- Because ReFS was designed not to fail, if failure does occur there are no tools provided to repair it. Third party tools are dependent on reverse engineering the system and (as at 2014) few of these exist.
Performance and competitor comparisons
In 2014, a review of ReFS and assessment of its readiness for production use, concluded that ReFS had key advantages over two of its main file system competitors, insofar as ZFS (used in *nix and FreeNAS) was widely criticized for its comparatively extreme memory requirements of many gigabytes of RAM, which ruled it out from a large number of medium and smaller systems, while offerings such as Drobo use proprietary methods which have no fallback if the company behind them fails.
ReFS was also found to be capable of running slightly faster than NTFS in most tests. However with integrity checking enabled, ReFS was found to be greatly slowed, and to run "dismally", suffering "a huge hit on performance and very high latency"; benchmark testing shows around 90% slowdown. However, they also point out that ReFS is still very much a newcomer ("essentially a “1.0” feature that is rough around the edges") and has not had the time to reach maturity that file systems such as ZFS have had.
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