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Last updated March 20, 2013 22:22, by cindys

ZFS Architecture Description

This page is designed to take you through a brief overview of the ZFS architecture. It is not intended as an introduction to ZFS. We assume that you already have some familiarity with common terms and definitions, as well as a general sense of file system architecture.

Traditionally, ZFS consists of three main components: ZPL (ZFS POSIX Layer), DMU (Data Management Unit), and SPA (Storage Pool Allocator) as indicated in the above image.

In this picture, you can see the three basic layers, though there are quite a few more elements in each. In addition, we show zvol consumers, as well as the management path, namely zfs(1M) and zpool(1M). You'll find a brief description of all these subsystems below. This is not intended to be an exhaustive overview of exactly how everything works. We hope that this summary tour is easy to follow. If not, feel free to post to

File System Consumers

These are the basic applications that interact with ZFS solely through the POSIX filesystem APIs. Virtually every application falls into this category. The system calls are passed through the generic OpenSolaris VFS layer to the ZPL.

Device Consumers

ZFS provides 'emulated volumes' or volumes or zvols. These volumes are backed by storage from a storage pool, but appear as a normal device under /dev. This is not a typical use case, but there are a small set of cases where this capability is useful. There are a small number of applications that interact directly with these devices, but the most common consumer is a kernel filesystem or target driver layered on top of the device.

Management GUI

A web-based ZFS GUI is available in Solaris 10 releases and on the ZFS storage appliance.

Management Consumers

These applications manipulate ZFS file systems or storage pools, including examining properties and dataset hierarchy. While there are some scattered exceptions (zoneadm, zoneadmd, fstyp), the two main applications are zpool(1M) and zfs(1M).


This library provides a Java interface to libzfs and is tailored specifically for the GUI. As such, it is geared primarily toward observability, as the GUI performs most actions through the CLI.


This is the primary interface for management apps to interact with the ZFS kernel module. The library presents a unified, object-based mechanism for accessing and manipulating both storage pools and file systems. The underlying mechanism used to communicate with the kernel is ioctl(2) calls through /dev/zfs.


The ZPL is the primary interface for interacting with ZFS as a file system. It is a (relatively) thin layer that sits atop the DMU and presents a filesystem abstraction of files and directories. It is responsible for bridging the gap between the VFS interfaces and the underlying DMU interfaces. It is also responsible for enforcing ACL (Access Control List) rules as well as synchronous (O_DSYNC) semantics.

ZVOL (ZFS Emulated Volume)

ZFS includes the ability to present raw devices backed by space from a storage pool. These are known as 'zvols' within the source code, and is implemented by a single file in the ZFS source.


This device is the primary point of control for libzfs. While consumers could consume the ioctl(2) interface directly, it is closely entwined with libzfs, and not a public interface (not that libzfs is, either). It consists of a single file, which does some validation on the ioctl() parameters and then vectors the request to the appropriate place within ZFS.

DMU (Data Management Unit)

The DMU is responsible for presenting a transactional object model, built atop the flat address space presented by the SPA. Consumers interact with the DMU via objsets, objects, and transactions. An objset is a collection of objects, where each object is an arbitrary piece of storage from the SPA. Each transaction is a series of operations that must be committed to disk as a group; it is central to the on-disk consistency for ZFS.

DSL (Dataset and Snapshot Layer)

The DSL aggregates DMU objsets into a hierarchical namespace, with inherited properties, as well as quota and reservation enforcement. It is also responsible for managing snapshots and clones of objsets.

ZAP (ZFS Attribute Processor)

The ZAP is built atop the DMU, and uses scalable hash algorithms to create arbitrary (name, object) associations within an objset. It is most commonly used to implement directories within the ZPL, but is also used extensively throughout the DSL, as well as a method of storing pool-wide properties. There are two very different ZAP algorithms, designed for different type of directories. The "micro zap" is used when the number of entries is relatively small and each entry is reasonably short. The "fat zap" is used for larger directories, or those with extremely long names.

ZIL (ZFS Intent Log)

While ZFS provides always-consistent data on disk, it follows traditional file system semantics where the majority of data is not written to disk immediately; otherwise performance would be pathologically slow. But there are applications that require more stringent semantics where the data is guaranteed to be on disk by the time the read(2) or write(2) call returns. For those applications requiring this behavior (specified with O_DSYNC), the ZIL provides the necessary semantics using an efficient per-dataset transaction log that can be replayed in event of a crash.


Traversal provides a safe, efficient, restartable method of walking all data within a live pool. It forms the basis of resilvering and scrubbing. It walks all metadata looking for blocks modified within a certain period of time. Thanks to the copy-on-write nature of ZFS, this has the benefit of quickly excluding large subtrees that have not been touched during an outage period. It is fundamentally a SPA facility, but has intimate knowledge of some DMU structures in order to handle snapshots, clones, and certain other characteristics of the on-disk format.

ARC (Adaptive Replacement Cache)

ZFS uses a modified version of an Adaptive Replacement Cache to provide its primary caching needs. This cache is layered between the DMU and the SPA and so acts at the virtual block-level. This allows filesystems to share their cached data with their snapshots and clones.

Pool Configuration (SPA)

While the entire pool layer is often referred to as the SPA (Storage Pool Allocator), the configuration portion is really the public interface. It is responsible for gluing together the ZIO and vdev layers into a consistent pool object. It includes routines to create and destroy pools from their configuration information, as well as sync the data out to the vdevs on regular intervals.

ZIO (ZFS I/O Pipeline)

The ZIO pipeline is where all data must pass when going to or from the disk. It is responsible for translation DVAs (Device Virtual Addresses) into logical locations on a vdev, as well as checksumming and compressing data as necessary. It is implemented as a multi-stage pipeline, with a bit mask to control which stage gets executed for each I/O.

VDEV (Virtual Devices)

The virtual device subsystem provides a unified method of arranging and accessing devices. Virtual devices form a tree, with a single root vdev and multiple interior (mirror and RAID-Z) and leaf (disk and file) vdevs. Each vdev is responsible for representing the available space, as well as laying out blocks on the physical disk.

LDI (Layered Driver Interface)

At the bottom of the stack, ZFS interacts with the underlying physical devices through LDI, the Layered Driver Interface, as well as the VFS interfaces (when dealing with files).

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