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ZFS Snapshots
Some of the workgroup-owned clusters and all the UD community clusters make use of ZFS to provide resilient shared storage space. ZFS is a Copy-On-Write (COW) storage technology, meaning that changes are always written to unused blocks in the underlying storage pool rather than overwriting blocks currently occupied by those entities. At any time a snapshot of the file system can be created very quickly and efficiently. Each snapshot references the blocks used at that point in time, and future changes naturally do not affect those blocks (thanks to COW).
The primary caveat to snapshots: they reference (consume) storage pool capacity that would otherwise get reused over time. Thus, it is often necessary to destroy older snapshots to make space available for new changes to the file system (the only other option being growing the underlying storage pool to add new capacity).
Each snapshot of a ZFS file system represents a point-in-time copy, making it an online backup copy of the file system. In many instances, the same snapshots are periodically transmitted to a secondary (often off-site) server where they guarantee recoverability of the file system should the primary server suffer a catastrophic failure.
Accessing snapshots
Any user having access to the root mount point of a ZFS file system can see a list of available snapshots. For example, my home directory on the Caviness community cluster is backed by a ZFS file system. To display all available snapshots:
[frey@login00 ~]$ cd ~/.zfs/snapshot [frey@login00 snapshot]$ ls -1 20180925-1815 20181023-1815 20181120-1815 20181218-1815 20190108-1815 20190115-1815 20190122-1815 20190129-1815 : 20190314-0615 20190314-1815 20190315-0615