ZFS Quick Reference Guide

ZFS is a combined file system and logical volume manager designed by Sun Microsystems. The features of ZFS include protection against data corruption, support for high storage capacities, efficient data compression, integration of the concepts of filesystem and volume management, snapshots and copy-on-write clones, continuous integrity checking and automatic repair, RAID-Z and native NFSv4 ACLs.

The ZFS file system is a new kind of file system that fundamentally changes the way file systems are administered, with the below mentioned features:

ZFS Pooled Storage

ZFS uses the concept of storage pools to manage physical storage. Historically, file systems were constructed on top of a single physical device. To address multiple devices and provide for data redundancy, the concept of a volume manager was introduced to provide a representation of a single device so that file systems would not need to be modified to take advantage of multiple devices. This design added another layer of complexity and ultimately prevented certain file system advances because the file system had no control over the physical placement of data on the virtualized volumes.

ZFS eliminates volume management altogether. Instead of forcing you to create virtualized volumes, ZFS aggregates devices into a storage pool

Transactional Semantics

ZFS is a transactional file system, which means that the file system state is always consistent on disk. In Transactional file system data is managed using copy on write semantics. Data is never overwritten, and any sequence of operations is either entirely committed or entirely ignored. Thus, the file system can never be corrupted through accidental loss of power or a system crash. Although the most recently written pieces of data might be lost, the file system itself will always be consistent. In addition, synchronous data (written using the O_DSYNC flag) is always guaranteed to be written before returning, so it is never lost.

Checksums and Self-Healing Data

With ZFS, all data and metadata is verified using a user-selectable checksum algorithm. In addition, ZFS provides for self-healing data. ZFS supports storage pools with varying levels of data redundancy. When a bad data block is detected, ZFS fetches the correct data from another redundant copy and repairs the bad data, replacing it with the correct data.

Unparalleled Scalability

ZFS is 128 bit filesystem, that allows 256 quadrillion zettabytes of storage.All metadata is allocated dynamically, so no need exists to preallocate inodes or otherwise limit the scalability of the file system when it is first created. All the algorithms have been written with scalability in mind. Directories can have up to 248 (256 trillion) entries, and no limit exists on the number of file systems or the number of files that can be contained within a file system.

ZFS Snapshots

A snapshot is a read-only copy of a file system or volume. Snapshots can be created quickly and easily. Initially, snapshots consume no additional disk space within the pool.

As data within the active dataset changes, the snapshot consumes disk space by continuing to reference the old data. As a result, the snapshot prevents the data from being freed back to the pool.

I've put together another post which lists a number of ZFS command line examples, click here to read it.