ZFS compared to most other file systems
Historically, the management of stored data has involved two aspects — the physical management of block devices such as hard drives and SD cards, and devices such as RAID controllers that present a logical single device based upon multiple physical devices (often undertaken by a volume manager, array manager, or suitable device driver), and the management of files stored as logical units on these logical block devices (a file system).
- 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 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 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, 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 error 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”. 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 the file system at previous points in time viewed, even on very large file systems, leading to “tremendous” savings in comparison to formal backup and restore processes, or cloned “on the spot” to form new independent file systems.