FreeNAS supports the OpenZFS filesystem which provides data integrity checking to prevent data corruption, enable point in time snapshotting, replication and several levels of redundancy including striping, mirroring, striped mirrors (RAID 1+0), and three levels of RaidZ.

The management of stored data generally involves two aspects: the physical volume management of one or more block storage devices such as hard drives and SD cards and their organization into logical block devices as seen by the operating system (often involving a volume manager, RAID controller, array manager, or suitable device driver), and the management of data and files that are stored on these logical block devices (a file system or other data storage).

Example: A RAID array of 2 hard drives and an SSD caching disk is controlled by Intel’s RST system, part of the chipset and firmware built into a desktop computer. The Windows user sees this as a single volume, containing an NTFS-formatted drive of their data, and NTFS is not necessarily aware of the manipulations that may be required (such as reading from/writing to the cache drive or rebuilding the RAID array if a disk fails). The management of the individual devices and their presentation as a single device is distinct from the management of the files held on that apparent device.

ZFS is unusual, because unlike most other storage systems, it unifies both of these roles and acts as both the volume manager and the file system. Therefore, it has complete knowledge of both the physical disks and volumes (including their condition and status, their logical arrangement into volumes, and also of all the files stored on them). ZFS is designed to ensure (subject to suitable hardware) that data stored on disks cannot be lost due to physical errors or misprocessing by the hardware or operating system, or bit rot events and data corruption which may happen over time, and its complete control of the storage system is used to ensure that every step, whether related to file management or disk management, is verified, confirmed, corrected if needed, and optimized, in a way that storage controller cards and separate volume and file managers cannot achieve.

ZFS also includes a mechanism for snapshots and replication, including snapshot cloning; the former is described by the FreeBSD documentation as one of its “most powerful features”, having features that “even other file systems with snapshot functionality lack”.^[15] Very large numbers of snapshots can be taken, without degrading performance, allowing snapshots to be used prior to risky system operations and software changes, or an entire production (“live”) file system to be fully snapshotted several times an hour, in order to mitigate data loss due to user error or malicious activity. Snapshots can be rolled back “live” or previous file system states can be viewed, even on very large file systems, leading to savings in comparison to formal backup and restore processes.^[15] Snapshots can also be cloned to form new independent file systems.