Chapter19.The Z File System (ZFS)

Written by Tom Rhodes, Allan Jude, Benedict Reuschling and Warren Block.

The Z File System, or ZFS, is an advanced file system designed to overcome many of the major problems found in previous designs.

Originally developed at Sun™, ongoing open source ZFS development has moved to the OpenZFS Project.

ZFS has three major design goals:

  • Data integrity: All data includes a checksum of the data. When data is written, the checksum is calculated and written along with it. When that data is later read back, the checksum is calculated again. If the checksums do not match, a data error has been detected. ZFS will attempt to automatically correct errors when data redundancy is available.

  • Pooled storage: physical storage devices are added to a pool, and storage space is allocated from that shared pool. Space is available to all file systems, and can be increased by adding new storage devices to the pool.

  • Performance: multiple caching mechanisms provide increased performance. ARC is an advanced memory-based read cache. A second level of disk-based read cache can be added with L2ARC, and disk-based synchronous write cache is available with ZIL.

A complete list of features and terminology is shown in Section19.8, “ZFS Features and Terminology”.

19.1.What Makes ZFS Different

ZFS is significantly different from any previous file system because it is more than just a file system. Combining the traditionally separate roles of volume manager and file system provides ZFS with unique advantages. The file system is now aware of the underlying structure of the disks. Traditional file systems could only be created on a single disk at a time. If there were two disks then two separate file systems would have to be created. In a traditional hardware RAID configuration, this problem was avoided by presenting the operating system with a single logical disk made up of the space provided by a number of physical disks, on top of which the operating system placed a file system. Even in the case of software RAID solutions like those provided by GEOM, the UFS file system living on top of the RAID transform believed that it was dealing with a single device. ZFS's combination of the volume manager and the file system solves this and allows the creation of many file systems all sharing a pool of available storage. One of the biggest advantages to ZFS's awareness of the physical layout of the disks is that existing file systems can be grown automatically when additional disks are added to the pool. This new space is then made available to all of the file systems. ZFS also has a number of different properties that can be applied to each file system, giving many advantages to creating a number of different file systems and datasets rather than a single monolithic file system.