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:
The destination path of the backing store files must have enough free space to hold the file system data. The size of the backing store files vary with the amount of activity on the file system.
The backing store file location must be different from the file system that is being captured in a snapshot.
The backing-store files can reside on any type of file system, including another UFS file system or an NFS file system.
Multiple backing-store files are created when you create a snapshot of a UFS file system that is larger than 512 Gbytes.
Backing-store files are sparse files. The logical size of a sparse file, as reported by the ls command, is not the same as the amount of space that has been allocated to the sparse file, as reported by the du command.
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.
