Tip of the Day: Understanding ReFS’s COW

Today’s Tip…

When a file’s metadata is modified in NTFS, it is written over the older metadata. However, this is not how ReFS works. ReFS uses a new ‘Copy On Write’ (COW) feature. Instead of overwriting old metadata, new metadata is written to a different location on disk. Once ReFS confirms that the new metadata is complete, pointers to the old metadata are modified to reference the new location.

Additionally, this feature can be activated for data streams as well. This is turned on by default when ReFS is on a Store Space that is configured as a mirror.

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