ZFS and Traditional File System Differences

This chapter discusses some significant differences between ZFS and traditional file systems. Understanding these key differences can help reduce confusion when using traditional tools to interact with ZFS.

The following sections are provided in this chapter:

ZFS File System Granularity

Historically, file systems have been constrained to one device so that the file systems themselves have been constrained to the size of the device. Creating and re-creating traditional file systems because of size constraints are time-consuming and sometimes difficult. Traditional volume management products helped manage this process.

Because ZFS file systems are not constrained to specific devices, they can be created easily and quickly, similar to the way directories are created. ZFS file systems grow automatically within the space allocated to the storage pool.

Instead of creating one file system, such as /export/home, to manage many user subdirectories, you can create one file system per user. In addition, ZFS provides a file system hierarchy so that you can easily set up and manage many file systems by applying properties that can be inherited by file systems contained within the hierarchy.

For an example of creating a file system hierarchy, see Creating a ZFS File System Hierarchy.

ZFS Space Accounting

ZFS is based on a concept of pooled storage. Unlike typical file systems, which are mapped to physical storage, all ZFS file systems in a pool share the available storage in the pool. So, the available space reported by utilities such as df might change even when the file system is inactive, as other file systems in the pool consume or release space. Note that the maximum file system size can be limited by using quotas. For information about quotas, see Setting Quotas on ZFS File Systems. Space can be guaranteed to a file system by using reservations. For information about reservations, see Setting Reservations on ZFS File Systems. This model is very similar to the NFS model, where multiple directories are mounted from the same file system (consider /home).

All metadata in ZFS is allocated dynamically. Most other file systems pre-allocate much of their metadata. As a result, an immediate space cost at file system creation for this metadata is required. This behavior also means that the total number of files supported by the file systems is predetermined. Because ZFS allocates its metadata as it needs it, no initial space cost is required, and the number of files is limited only by the available space. The output from the df -g command must be interpreted differently for ZFS than other file systems. The total files reported is only an estimate based on the amount of storage that is available in the pool.

ZFS is a transactional file system. Most file system modifications are bundled into transaction groups and committed to disk asynchronously. Until these modifications are committed to disk, they are termed pending changes. The amount of space used, available, and referenced by a file or file system does not consider pending changes. Pending changes are generally accounted for within a few seconds. Even committing a change to disk by using fsync(3C) or O_SYNC does not necessarily guarantee that the space usage information is updated immediately.

Out of Space Behavior

File system snapshots are inexpensive and easy to create in ZFS. Most likely, snapshots will be common in most ZFS environments. For information about ZFS snapshots, see Working With ZFS Snapshots and Clones.

The presence of snapshots can cause some unexpected behavior when you attempt to free space. Typically, given appropriate permissions, you can remove a file from a full file system, and this action results in more space becoming available in the file system. However, if the file to be removed exists in a snapshot of the file system, then no space is gained from the file deletion. The blocks used by the file continue to be referenced from the snapshot.

As a result, the file deletion can consume more disk space, because a new version of the directory needs to be created to reflect the new state of the namespace. This behavior means that you can get an unexpected ENOSPC or EDQUOT when attempting to remove a file.

Mounting ZFS File Systems

ZFS is designed to reduce complexity and ease administration. For example, with existing file systems you must edit the /etc/vfstab file every time you add a new file system. ZFS has eliminated this requirement by automatically mounting and unmounting file systems according to the properties of the dataset. You do not need to manage ZFS entries in the /etc/vfstab file.

For more information about mounting and sharing ZFS file systems, see Mounting and Sharing ZFS File Systems.

Traditional Volume Management

As described in ZFS Pooled Storage, ZFS eliminates the need for a separate volume manager. ZFS operates on raw devices, so it is possible to create a storage pool comprised of logical volumes, either software or hardware. This configuration is not recommended, as ZFS works best when it uses raw physical devices. Using logical volumes might sacrifice performance, reliability, or both, and should be avoided.

The NFSv4 ACL Model

Older versions of Solaris supported an ACL implementation that was primarily based on the POSIX-draft specification. The POSIX-draft based ACLs are used to protect UFS files, while a new ACL model based on the NFSv4 specification is used to protect ZFS files.

The main differences of this ACL model are:

For more information about using ACLs with ZFS files, see Using ACLs to Protect ZFS Files.