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PowerScale OneFS 9.2.1.0 CLI Administration Guide

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Shadow-store considerations

Shadow stores are hidden files that are referenced by cloned and deduplicated files. Files that reference shadow stores behave differently than other files.

  • Reading shadow-store references might be slower than reading data directly. Reading noncached shadow-store references is slower than reading noncached data. Reading cached shadow-store references takes no more time than reading cached data.
  • When files that reference shadow stores are replicated to another PowerScale cluster or backed up to a Network Data Management Protocol (NDMP) backup device, the shadow stores are not transferred to the target PowerScale cluster or backup device. The files are transferred as if they contained the data that they reference from shadow stores. On the target PowerScale cluster or backup device, the files consume the same amount of space as if they had not referenced shadow stores.
  • When OneFS creates a shadow store, OneFS assigns the shadow store to a storage pool of a file that references the shadow store. If you delete the storage pool that a shadow store resides on, the shadow store is moved to a pool that contains another file that references the shadow store.
  • OneFS does not delete a shadow-store block immediately after the last reference to the block is deleted. Instead, OneFS waits until the ShadowStoreDelete job is run to delete the unreferenced block. If many unreferenced blocks exist on the cluster, OneFS might report a negative deduplication savings until the ShadowStoreDelete job is run.
  • Shadow stores are protected at least as much as the most protected file that references it. For example, if one file that references a shadow store resides in a storage pool with +2 protection and another file that references the shadow store resides in a storage pool with +3 protection, the shadow store is protected at +3.
  • Quotas account for files that reference shadow stores as if the files contained the data that is referenced from shadow stores. From the perspective of a quota, shadow-store references do not exist. However, if a quota includes data protection overhead, the quota does not account for the data protection overhead of shadow stores.

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