|Full name||Resilient File System|
|Introduced||1 August 2012 with Windows Server 2012|
|Directory contents||B+ tree|
|Max. volume size||1 yobibyte (280 bytes)|
|Max. file size||16 exbibytes (264−1 bytes)|
|Data deduplication||Yes, since v3.2 debuting in 2016 v1709|
|Supported operating systems|
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 problems that had become significant over the years since NTFS was conceived, which are related to how data storage requirements had changed. The key design advantages of ReFS include automatic integrity checking and data scrubbing, removal of the need for running chkdsk, protection against data degradation, built-in handling of hard disk drive failure and redundancy, integration of RAID functionality, a switch to copy/allocate on write for data and metadata updates, handling of very long paths and filenames, and storage virtualization and pooling, including almost arbitrarily sized logical volumes (unrelated to the physical sizes of the used drives).
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; this was achieved as of Windows 8.1. The initial versions removed some NTFS features, such as disk quotas, alternate data streams, and extended attributes. Some of these were re-implemented in later versions of ReFS.
In early versions (2012–2013), ReFS was similar to 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. Pre-release concerns were also voiced by one blogger[who?] over Storage Spaces, the storage system designed to underpin ReFS, which reportedly could fail in a manner that prevented ReFS from recovering automatically.
Feature changes compared to NTFS
Major new features
- Improved reliability for on-disk structures
- ReFS uses B+ trees for all on-disk structures, including all 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 exbibytes (264−1 bytes), a maximum of 18.4 × 1018 directories and a maximum volume size of 1 yobibyte (280 bytes) (with 64 KiB clusters) which allows large scalability with no practical limits on file and directory size (hardware limits 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 have 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 for file data; this is controlled by a new "integrity" attribute applicable to both files and directories. If file data or metadata become corrupt, the file can be deleted without taking the whole volume offline for maintenance, and then be 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 a data scrubbing process, which periodically reads all mirror copies and verifies their checksums, then replaces bad copies with good ones.
Some NTFS features are not implemented in ReFS. These include object IDs, 8.3 filename, NTFS compression, Encrypting File System (EFS), transactional NTFS, hard links, extended attributes, and disk quotas. 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. Data deduplication was missing in early versions of ReFS., it was implemented in v3.2, debuting in Windows Server v1709.
Support for alternate data streams was initially not implemented in ReFS. In Windows 8.1 64-bit and Server 2012 R2 the file system reacquired support for alternate data streams, with lengths of up to 128K, and automatic correction of corruption when integrity streams are used on parity spaces. ReFS had initially been unsuitable for Microsoft SQL Server instance allocation due to the absence of alternate data streams.
As of March 2015, a review of the state of ReFS on WIndowsNetworking.com stated:
You can’t (at least at this time) boot Windows from an ReFS volume and the first versions of ReFS don’t include file-level compression and encryption, disk quotas or hard links, all of which are advantages of NTFS over the FAT file systems. Note that ReFS does support sparse files, reparse points, case-sensitive file names and Unicode in file names and perhaps most important, it preserves and enforces access control lists (ACLs).
It’s obvious that ReFS in its current iteration is not a replacement for NTFS ... because some applications that rely on specific NTFS features might not work with ReFS [... however...] Storage of most conventional data doesn’t require the specific NTFS features that aren’t supported by ReFS and so ReFS can handle that duty nicely. Its primary use case is on file servers that store extremely large amounts of data. It has data integrity and recovery mechanisms built into the file system, as well. That means those tools that are designed to detect and repair file corruption in other file systems aren’t necessary, so their incompatibility with ReFS isn’t really an issue. Additionally, although ReFS doesn’t support file level (Encrypting File System) encryption, BitLocker can be used to protect ReFS volumes so that’s not so much of an issue, either [...]
ReFS has some distinct advantages over current reigning Windows file system NTFS, but it also has some drawbacks. It boasts self-healing powers, ability to repair files without down time, less risk that data will be lost when there’s a power failure (due to the way it writes metadata), and of course the ability to create huge volumes and files and even give those files names that are longer than 255 characters if you wish. But it’s not quite ready for prime time yet.
Version history and compatibility
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ReFS has some different versions, with various degrees of compatibility between operating system versions. Aside for development versions of the filesystem, usually later operating system versions can mount filesystems created with earlier OS versions (backwards compatibility). Some features may not be compatible with the feature set of the OS. The version, cluster size and other features of the filesystem can be queried with the command fsutil fsinfo refsinfo volumename.
- v1.1: The original version, formatted by Windows Server 2012.
- 1.2: Default version if formatted by Windows 8.1, Windows 10 v1507 to v1607, Windows Server 2012 R2, and when specified ReFSv1 on Windows Server 2016. Can use alternate data streams under Windows Server 2012 R2.
- 22.2: Default version formatted by Windows 10 Preview build 10049 or earlier. Could not be mounted in 10061 and later.
- 2.0: Default version formatted by Windows Server 2016 TP2 and TP3. Could not be mounted in Windows 10 Build 10130 and later, or Windows Server 2016 TP4 and later.
- 3.0: Default version formatted by Windows Server 2016 TP4 and TP5.
- 3.1: Default version formatted by Windows Server 2016 RTM.
- 3.2: Default version formatted by Windows 10 v1703 and Windows Server Insider Preview build 16237. Can be formatted with Windows 10 Insider Preview 15002 or later (though only became the default somewhere between 15002 and 15019). Supports deduplication in the server version.
- 3.3: Default version formatted by Windows 10 Enterprise v1709 (Creation ReFS volume ability was removed from Home and Pro starting at build 16226 – read/write ability remains) and Windows Server version 1709 (starting with Windows 10 Enterprise Insider Preview build 16257 and Windows Server Insider Preview build 16257).
v1507 – v1607
- 1: The following message is recorded to the event log: 'Volume "?:" was mounted in an older version of Windows. Some features may be lost.'
- 2: Windows upgrades it to 3.1 when the volume is mounted with write access.
- 3: Windows upgrades it to 3.2 when the volume is mounted with write access.
- 4: Windows upgrades it to 3.3 when the volume is mounted with write access.
Stability and known problems
Issues identified or suggested for ReFS, when running on Storage Spaces (its intended design), include:
- 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 became 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.
- Even when Storage Spaces is not thinly provisioned, ReFS may still be unable to dependably correct all file errors in some situations, because Storage Spaces operates on blocks and not files, and therefore some files may potentially lack necessary blocks or recovery data if part of the storage space is not working correctly. As a result, disk and data addition and removal may be impaired, and redundancy conversion becomes difficult or impossible.
- 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 of 2014) few of these exist.
- Windows Store cannot install apps on a ReFS volume.
Server 2016 updates
At the Storage Developer Conference 2015, a Microsoft developer presented enhancements of ReFS expected to be released with Windows Server 2016 and included in Technical Preview 4, titled "ReFS v2". It highlighted that ReFS now included capabilities for very high speed moving, reordering, and cloning of blocks between files (which can be done for all blocks of a file). This is particularly needed for virtualization, and is stated to allow fast provisioning, diff merging, and tiering. Other enhancements cover the redo log (for synchronous disk writes), parallelization, efficient tracking of uninitialized sparse data and files, and efficient 4k I/O. ReFS with File Integrity enabled also acts more like a log-structured file system, coalescing small random writes into large sequential ones for efficiency.[dubious ]
Performance and competitor comparisons
Other operating systems have competing file systems to ReFS, of which the best known are ZFS and Btrfs, in the sense that all three are designed to integrate data protection, snapshots, and silent high speed background healing of corruption and data errors.
In 2012, Phoronix wrote an analysis of ReFS vs Btrfs, a copy-on-write file system for Linux. Their features are similar, with both supporting checksums, RAID-like use of multiple disks, and error detection/correction. However, ReFS lacks copy-on-write snapshots and compression, both found in Btrfs and ZFS.
In 2014, a review of ReFS and assessment of its readiness for production use concluded that ReFS had at least some advantages over two of its main file system competitors.
- ZFS (used in Solaris, illumos, FreeBSD and others) was widely criticized for its comparatively extreme memory requirements of many gigabytes of RAM for online deduplication, which ruled it out from a large number of medium and smaller systems. However, turning off online deduplication on ZFS, as this feature is unsupported in ReFS, yields a more even comparison between the two file systems since ZFS then has a memory requirement of only a few hundred megabytes.
- Offerings such as Drobo used proprietary methods which have no fallback if the company behind them fails.
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