What Is ReFS (Resilient File System)?
Windows Server 2012 originally introduced Microsoft’s new ReFS file system. The ReFS file system included on Windows 10, where it can only be used as part of the drive-pooling Storage Spaces feature.
Short for “Resilient File System”, ReFS is a new file system built using code from the current NTFS file system. At the moment, ReFS is not just a replacement for NTFS. It has its own advantages and disadvantages.
As ReFS is Microsoft’s newest file system, it’s designed to address a few major issues with NTFS. ReFS is designed to be more resilient against data corruption, perform better for certain workloads, and scale better for very large file systems.
Storage Spaces make it easy to cope from a failed disk, but are no help if a disk is merely producing bad data. The Storage Space will be able to tell you if two mirrored drives differ or if the parity check fails, but have no way of determining which drive is right and which is wrong. Erasures, where the data is missing altogether, can be corrected; errors, where the data is wrong, can only be detected.
ReFS is designed to pick up where Storage Spaces leave off. To protect its internal data structures, file system metadata, and, optionally, user data against corruption, ReFS calculates and stores checksums for the data and metadata. Each piece of information protected by the checksum is fed into a checksum algorithm, and the result is a number, the checksum; in ReFS's case, the checksum is a 64-bit number. Checksum algorithms are designed such that a small change in the input causes a large change in the resulting checksum.
Every time ReFS reads file system metadata (or data that has opted in to the checksum protection) it will compute the checksum for the information it has read, and compare this against the stored value. If the two are in agreement then the data has been read correctly; if they aren't, it hasn't.
This checksum protection guards against a range of problems. When writing, two particular issues are "lost writes," where the data never makes it to disk, and "misdirected writes," where a bug in either the file system driver or the firmware of the drive or its controller causes a write to go to the wrong location on disk. When reading data, the biggest concern is "bit rot"—the corruption of correctly-written data due to failures of the disk's magnetic storage.
This solves the problem of not knowing which side of a mirrored pair is the correct one. When used with a Storage Spaces mirror, ReFS will test the checksums of each side of the mirror independently, and then use this to determine which one is correct and which is not.
To further improve protection against bit rot, ReFS will perform scrubbing: it will periodically read all the data and metadata in a volume, verify the checksums are correct, and if necessary use mirrored copies to repair the bad data. This feature is available on both Windows 10 and Windows 8.1.
ReFS is also designed to provide greater protection against power failures during write operations.
ReFS doesn’t just check files for corruption when reading and writing them. An automated data integrity scanner regularly checks all files on the drive to identify and fix data corruption, too. It’s an auto-correcting file system. You don’t need to use chkdsk at all.
ReFS is more modern than NTFS, and supports much larger volumes and longer file names. In the long term, these are important improvements.
On an NTFS file system, file paths are limited to 255 characters. With ReFS, a file name can be up to 32768 characters long. Windows 10 now allows you to disable this short character limit for NTFS file systems, but it’s always disabled on ReFS volumes.
ReFS also discards the DOS-style 8.3 filenames. On an NTFS volume, you can still access C:\Program Files\ at C:\PROGRA~1\ for compatibility purposes with old software. These legacy file names are gone on ReFS.
NTFS has a theoretical maximum volume size of 16 exabytes, while ReFS has a theoretical maximum volume size of 262144 exabytes. That doesn’t matter much right now, but it will one day.
When used with Storage Spaces, ReFS supports “real-time tier optimization”. You could have a drive pool with both drives optimized for performance and drives optimized for capacity. ReFS will always write to the drives in the performance tier, maximizing performance. In the background, ReFS will automatically move large chunks of data to the slower drives for long term storage.
On Windows Server 2016, Microsoft improved ReFS to offer better performance with certain virtual machine features. Microsoft’s own Hyper-V virtual machine software takes advantage of these (and in theory, other virtual machine software could support them if they wanted to).
For example, ReFS supports block cloning, which accelerates virtual machine cloning and checkpoint-merging operations.
ReFS also offers a new “sparse VDL” feature that allows ReFS to quickly write zeros to a large file. This makes it much faster to create a new, empty, fixed-sized virtual hard disk (VHD) file. Whereas this might take 10 minutes with NTFS, it can take just a few seconds with ReFS.
You can’t just switch to ReFS from NTFS. Windows cannot boot from a ReFS file system, and requires NTFS.
ReFS also omits other features NTFS includes, including file system compression and encryption, hard links, extended attributes, data deduplication, and disk quotas. However, ReFS is compatible with a variety of features. For example, while you can’t perform encryption of specific data at the file system level, ReFS is compatible with full-disk BitLocker encryption.
You can currently only use ReFS with Storage Spaces, where its reliability features help protect against data corruption. On Windows Server 2016, you can choose to format volumes with ReFS instead of NTFS. You might want to do this for a volume you plan on storing virtual machines on, for example. However, you still can’t use ReFS on your boot volume. Windows can only boot from an NTFS drive.
On Windows 10, you can only use ReFS through the Storage Spaces feature. You’ll automatically benefit from the data integrity features in the ReFS file system if you choose it.
On Windows Server, you can choose to format some volumes as ReFS using the normal disk management tools, which may be particularly useful if you’re using virtual machines on those drives. But you can’t format your boot drive as ReFS, and you’ll lose access to some NTFS features.
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