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Microsoft's ReFS Filesystem for Windows 8 Explained

In 2010 Microsoft announced Drive Extender v2, which was to be their next generation file system and disk pooling technology for Windows Home Server 2011. As planned, Drive Extender v2 would have been an extremely aggressive modernization of how storage worked for Windows Home Server, coupling block based storage with distributed file slices and ECC hashing all in the name of expandability and resiliency. Unfortunately Drive Extender v2 was apparently a bit too aggressive, and was canned towards the end of 2010. Fast forward to 2012 and Microsoft has begun reassembling the band for Windows 8. Two weeks ago Storage Spaces was announced, in essence reviving much of the Drive Extender feature set. Storage Spaces allows for pooling of drives together into one (or more) pools, along with RAID1/5 style redundancy for recovering from drive failures. Storage Spaces on its own is meant to be rather resilient, but as it turns out Storage Spaces is just one part of a two part solution. Microsoft has also been working on creating a successor to the underlying file system, and that’s the subject of today’s Building Windows 8 Blog. NTFS’s heir-apparent is the Resilient File System (ReFS), which if the name doesn’t tip you off, is all about adding yet more resiliency to file storage under Windows. Whereas Storage Spaces is about operating at a file/disk level, ReFS operates at a file/block level, concerning itself with the organization of the file system and how to handle errors within individual files. To do that, Microsoft is effectively replacing everything NTFS below the API itself. Compared to ZFS, Drive Extender v2, and other next-generation file systems, ReFS’s feature set covers most of what you’d expect to find. At a most practical level, by dropping NTFS some of NTFS’s most annoying quirks have finally been eradicated, including the 255 character limit for file and path names. At the same time chkdsk will be far less visible as ReFS’s salvage feature will allow Windows to load a file system and take offline only the affected files and not the entire file system, thereby allowing many file system fix operations to be handled online instead of offline. But of course the real work is in adding resiliency, where MS has added a number of features. File metadata is now checksummed so that Windows can identify silently corrupted metadata, and metadata is written in an allocate-on-write style (rather than overwriting itself) so that a torn write cannot occur. This same level of checksumming can also be extended to files themselves through what Microsoft is calling Integrity Streams, which when coupled with Storage Spaces will allow Windows to identify copies that have become silently corrupted (through bit rot and other modes of failure) and replace them with a known good copy. In fact the only DEv2 resiliency feature that's not accounted for here is ECC error correction for files (as opposed to checksums), which Microsoft is apparently leaving up to Storage Spaces' mirroring features. Finally, as ReFS is part of Windows 8 it will be delivered starting with the Windows 8 beta next month, but as Microsoft is quick to note file system integrity is so paramount that they’re taking things very slowly even with the extensive testing they’re doing. Officially ReFS will be production ready when Windows 8 is released, where it will ship enabled on Windows 8 Server for use with storage volumes, and at which point Microsoft is hoping the real-world usage on file servers will further shake out any bugs in ReFS and otherwise validate it for use. Once it’s passed a trial-by-fire with Windows 8 Server it will be rolled out to Windows 8 Client for use with storage volumes (effectively testing it on less reliable hardware), and then finally to both platforms for use on boot volumes. For an in-depth look at how this technology works (and a FAQ which answers, among other questions, some inquiries about the B+ tree structure of ReFS and what NTFS features won’t make the cut), check out the full post using the link below. Source: Building Windows 8 Blog

Read more: AnandTech

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