Microsoft has given its first detailed description of ReFS, the new file system that will gradually succeed the now-venerable NTFS in Windows systems.
The Resilient File System or ReFS, the successor to NTFS, will make its first appearance in Windows Server 8.
The Resilient File System (ReFS) will make its first appearance asa storage system in Windows Server 8, after which it will evolve intoa system for client storage, then ultimately for boot volumes. OnMonday, Microsoft used a poston its Building Windows 8 blog to lay out details on ReFS.
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“Along with StorageSpaces, ReFS forms the foundation of storage on Windows for thenext decade or more,” Microsoft development manager Surendra Vermawrote. “We believe this significantly advances our state of the artfor storage… we expect that we will see ReFS as the next massivelydeployed file system.”
The “staged evolution” of ReFS, beginning with its use as a fileserver, is the same approach Microsoft has taken with previous filesystems.
According to Verma, ReFS has many of the same features andsemantics as NTFS, which was introduced in 1993. These include “BitLocker encryption, access-control lists for security, USN journal, change notifications, symbolic links, junction points, mount points, reparse points, volume snapshots, file IDs and OpLocks“.
If a file-access API can access an NTFS volume, it will be able toaccess data stored on ReFS.
“Rewriting the code that implements file system semantics would notlead to the right level of compatibility, and the issues introducedwould be highly dependent on application code, call timing andhardware. Therefore in building ReFS, we reused the code responsiblefor implementing the Windows file system semantics,” Verma said.
On-disk storage engine
What has changed is the on-disk storage engine underneath thereused code. The ReFS engine exclusively uses the so-called B+ tree structure torepresent stored information. Verma indicated this will mean a simplersystem, with the choice of structure designed to be as scalable aspossible.
Along with Storage Spaces, ReFS forms the foundation of storage on Windows for the next decade or more.
– Surendra Verma, Microsoft
On the subject of detecting and fixing corruption, Verma explainedthat all ReFS data is check-summed at the level of the B+ tree page, with the checksum — an algorithm for checking data integrity — being stored away from that page.
“This allows us to detect all forms of disk corruption, includinglost and misdirected writes and bit rot (degradation of data on themedia). In addition, we have added an option where the contents of afile are check-summed as well,” Verma wrote.
The Storage Spaces feature will be available for NTFS and, unlikeReFS, for client PCs from the start. It is supposed to improveperformance and protect data by maintaining copies on multipledisks.
When ReFS is used alongside mirrored Storage Spaces, corruptionswill be “automatically and transparently fixed”, Verma added, notingthat ReFS also had a ‘salvage’ feature for making sure thatnon-repairable corruption does not affect the availability of “gooddata”.
Scrubbing
ReFS offers a new way of dealing with ‘bit rot’, or data decay, inthat it uses a system task to “periodically [scrub] all metadata andIntegrity Stream data on a ReFS volume residing on a mirrored StorageSpace”, Verma said.
“Scrubbing involves reading all the redundant copies and validatingtheir correctness using the ReFS checksums. If checksums mismatch, badcopies are fixed using good ones,” he said, adding that “the fileattribute FILE_ATTRIBUTE_NO_SCRUB_DATA indicates that the scrubbershould skip the file”.
Responding to readers’ questions about the blogpost, Verma alsostressed that Windows 7 users would be able to read ReFS-formattedpartitions from a Windows 8 Server, either by using a new file systemdriver or by sharing a folder out from the server.
“Note that support for NTFS is going to be present in Windows forthe foreseeable future, so you should always be able to access allyour NTFS data across versions without any problems,” he added.
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