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SnapshotIQ uses both copy on write (CoW) and redirect on write (RoW) strategies for its differential snapshots and uses the most appropriate method for a given situation. Both have pros and cons, and OneFS dynamically picks which flavor to use in order to maximize performance and keep overhead to a minimum.
With copy on write, as the name suggests, a new write to HEAD results in the old blocks being copied out to the snapshot version first. Although this incurs a double write penalty, it results in less fragmentation of the HEAD file, which is better for cache prefetch. Typically, CoW is most prevalent in OneFS and is primarily used for small changes, inodes, and directories.
Redirect on write, on the other hand, avoids the double write penalty by writing changes to a snapshot protected file directly to another free area of the file system. However, the flip side to this is increased file fragmentation. Because file contiguity is not maintained by virtue of writing changes to other file system regions, RoW in OneFS is used for more substantial changes such as deletes and large sequential writes.
Note: There is no reserved space requirement for snapshots in OneFS. Snapshots can use as much or little of the available file system space as desirable. A snapshot reserve can be configured, if preferred.