System Administration Guide: Basic Administration

UFS Snapshots Overview

The Solaris release includes the fssnap command for backing up file systems while the file system is mounted. You can use the fssnap command to create a read-only snapshot of a file system. A snapshot is a file system's temporary image that is intended for backup operations.

When the fssnap command is run, it creates a virtual device and a backing-store file. You can back up the virtual device, which looks and acts like a real device, with any of the existing Solaris backup commands. The backing-store file is a bitmapped file that contains copies of pre-snapshot data that has been modified since the snapshot was taken.

Why Use UFS Snapshots?

UFS snapshots enables you to keep the file system mounted and the system in multiuser mode during backups. Previously, you were advised to bring the system to single-user mode to keep the file system inactive when you used the ufsdump command to perform backups. You can also use additional Solaris backup commands like tar and cpio to back up a UFS snapshot for more reliable backups.

The fssnap command gives administrators of non-enterprise-level systems the power of enterprise-level tools like Sun StorEdge^TM Instant Image without the large storage demands.

UFS snapshots is similar to the Instant Image product. Instant Image allocates space equal to the size of the entire file system that is being captured. However, the backing-store file that is created by UFS snapshots occupies only as much disk space as needed, and you can place a maximum size on the backing-store file.

This table describes specific differences between UFS snapshots and Instant Image.

UFS Snapshots	Instant Image
Size of the backing-store file depends on how much data has changed since the snapshot was taken	Size of the backing-store file equivalent equals the size of the entire file system being copied
Does not persist across system reboots	Persists across system reboots
Works on UFS file systems	Cannot be used with root (`/`) or `/usr` file systems
Starting with the Solaris 8 1/01 release	Part of Sun StorEdge products

Although UFS snapshots can make copies of large file systems, Instant Image is better suited for enterprise-level systems. UFS snapshots is better suited for smaller systems.

UFS Snapshots Performance Issues

When the UFS snapshot is first created, users of the file system might notice a slight pause. The length of the pause increases with the size of the file system to be captured. While the snapshot is active, users of the file system might notice a slight performance impact when the file system is written to, but they will see no impact when the file system is read.