System Administration Guide: Devices and File Systems

UFS Snapshots Overview

You can use the fssnap command to back up file systems while the file system is mounted. This command to creates 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 bitmap file that contains copies of pre snapshot data that has been modified since the snapshot was taken.

Keep the following key points in mind when specifying backing-store files:

For more information about creating snapshots for a UFS file system larger than 512 Gbytes, see Creating a Multiterabyte UFS Snapshot.

Why Use UFS Snapshots?

The UFS snapshots feature provides additional availability and convenience for backing up a file system because the file system remains mounted and the system remains in multiuser mode during backups. Then, you can use the tar or cpio commands to back up a UFS snapshot to tape for more permanent storage. If you use the ufsdump command to perform backups, the system should be in single-user mode to keep the file system inactive when you perform backups.

The fssnap command gives administrators of non enterprise-level systems the power of enterprise-level tools, such as Sun StorEdgeTM Instant Image, without the large storage demands.

The UFS snapshots feature is similar to the Instant Image product. 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. 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.

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

UFS Snapshots 

Sun StorEdge 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 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

Available starting with the Solaris 8 1/01 release 

Part of Sun StorEdge products 

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. However, they see no impact when the file system is read.