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OneFS shadow stores are file system containers that allow data to be stored in a shareable manner. As such, files on OneFS can contain both physical data and pointers, or references, to shared blocks in shadow stores. Shadow stores were introduced in OneFS 7.0, initially supporting OneFS file clones, and there are many overlaps between cloning and deduplicating files. The other main consumer of shadow stores is OneFS Small File Storage Efficiency (SFSE) for archive. This feature maximizes the space utilization of a cluster by decreasing the amount of physical storage required to house a small file archive repository, such as a typical healthcare PACS dataset.
Shadow stores are similar to regular files but are hidden from the file system namespace, so cannot be accessed via a pathname. A shadow store typically grows to a maximum size of 2GB (or about 256K blocks), with each block able to be referenced by 32,000 files. If the reference count limit is reached, a new block is allocated, which may or may not be in the same shadow store. Additionally, shadow stores do not reference other shadow stores. And snapshots of shadow stores are not permitted because the data stored in shadow stores cannot be overwritten.