Chapter 24 Using UFS Snapshots (Tasks)

This chapter describes how to create and back up UFS snapshots.

For information on the procedures associated with creating UFS snapshots, see Using UFS Snapshots (Task Map).

For overview information about performing backups, see Chapter 22, Backing Up and Restoring File Systems (Overview).

Using UFS Snapshots (Task Map)

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 bitmap file that contains copies of presnapshot data that has been modified since the snapshot was taken.

Why Use UFS Snapshots?

The UFS snapshots feature 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, such as tar and cpio, to back up a UFS snapshot for more reliable backups.

The fssnap command gives administrators of nonenterprise-level systems the power of enterprise-level tools, such as Sun StorEdge^TM 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 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 see no impact when the file system is read.

Creating and Deleting UFS Snapshots

When you use the fssnap command to create a UFS snapshot, observe how much disk space the backing-store file consumes. The backing-store file uses no space, and then it grows quickly, especially on heavily used systems. Make sure that the backing-store file has enough space to expand. Or, limit its size with the -o maxsize=n [k,m,g] option, where n [k,m,g] is the maximum size of the backing-store file.

If the backing-store file runs out of space, the snapshot might delete itself, which causes the backup to fail. Check the /var/adm/messages file for possible snapshot errors.

For more information, see the fssnap_ufs(1M) man page.

How to Create a UFS Snapshot

Steps

1. Become superuser or assume an equivalent role.

2. Make sure that the file system has enough disk space for the backing-store file.

   # df -k

3. Make sure that a backing-store file of the same name and location does not already exist.

   # ls /backing-store-file

4. Create the UFS snapshot.

   # fssnap -F ufs -o bs=/backing-store-file /file-system

   ---

   Note –

   The backing-store file must reside on a different file system than the file system that is being captured using UFS snapshots.

   ---

5. Verify that the snapshot has been created.

   # /usr/lib/fs/ufs/fssnap -i /file-system

Example 24–1 Creating a UFS Snapshot

The following example shows how to create a snapshot of the /usr file system. The backing-store file is /scratch/usr.back.file. The virtual device is /dev/fssnap/1.

# fssnap -F ufs -o bs=/scratch/usr.back.file /usr
/dev/fssnap/1

The following example shows how to limit the backing-store file to 500 Mbytes.

# fssnap -F ufs -o maxsize=500m,bs=/scratch/usr.back.file /export/home 
/dev/fssnap/1

How to Display UFS Snapshot Information

You can display the current snapshots on the system by using the fssnap -i option. If you specify a file system, you see detailed information about that snapshot. If you don't specify a file system, you see information about all of the current UFS snapshots and their corresponding virtual devices.

Use the UFS file system-specific fssnap command to view the extended snapshot information as shown in the following examples.

Steps

1. Become superuser or assume an equivalent role.

2. List all current snapshots:

   For example:

   # /usr/lib/fs/ufs/fssnap -i Snapshot number : 0 Block Device : /dev/fssnap/0 Raw Device : /dev/rfssnap/0 Mount point : /usr Device state : idle Backing store path : /var/tmp/snapshot3 Backing store size : 256 KB Maximum backing store size : Unlimited Snapshot create time : Wed Oct 08 10:38:25 2003 Copy-on-write granularity : 32 KB Snapshot number : 1 Block Device : /dev/fssnap/1 Raw Device : /dev/rfssnap/1 Mount point : / Device state : idle Backing store path : /tmp/bs.home Backing store size : 448 KB Maximum backing store size : Unlimited Snapshot create time : Wed Oct 08 10:39:29 2003 Copy-on-write granularity : 32 KB

3. Display detailed information about a specific snapshot:

   For example:

   # /usr/lib/fs/ufs/fssnap -i /usr Snapshot number : 0 Block Device : /dev/fssnap/0 Raw Device : /dev/rfssnap/0 Mount point : /usr Device state : idle Backing store path : /var/tmp/snapshot3 Backing store size : 256 KB Maximum backing store size : Unlimited Snapshot create time : Wed Oct 08 10:38:25 2003 Copy-on-write granularity : 32 KB

Deleting a UFS Snapshot

When you create a UFS snapshot, you can specify that the backing-store file is unlinked. An unlinked backing-store file is removed after the snapshot is deleted. If you don't specify the -o unlink option when you create a UFS snapshot, you must delete the backing-store file manually.

The backing-store file occupies disk space until the snapshot is deleted, whether you use the -o unlink option to remove the backing-store file or you delete the file manually.

How to Delete a UFS Snapshot

You can delete a snapshot either by rebooting the system or by using the fssnap -d command. When you use this command, you must and specify the path of the file system that contains the UFS snapshot.

Steps

1. Become superuser or assume an equivalent role.

2. Identify the snapshot to be deleted.

   # /usr/lib/fs/ufs/fssnap -i

3. Delete the snapshot.

   # fssnap -d /file-system Deleted snapshot 1.

4. (Optional) If you did not use the -o unlink option when you created the snapshot, manually delete the backing-store file.

   # rm /file-system/backing-store-file

Example 24–2 Deleting a UFS Snapshot

The following example shows how to delete a snapshot and assumes that the -o unlink option was not used.

# fssnap -i
    0    /    1    /usr
# fssnap -d /usr
 Deleted snapshot 1.
# rm /scratch/usr.back.file

Backing Up a UFS Snapshot

You can create a full backup or an incremental backup of a UFS snapshot. You can use the standard Solaris backup commands to back up a UFS snapshot.

The virtual device that contains the UFS snapshot acts as a standard read-only device. So, you can back up the virtual device as if you were backing up a file system device.

If you are using the ufsdump command to back up a UFS snapshot, you can specify the snapshot name during the backup. See the following procedure for more information.

How to Create a Full Backup of a UFS Snapshot (ufsdump)

Steps

1. Become superuser or assume an equivalent role.

2. Identify the UFS snapshot to be backed up.

   # /usr/lib/fs/ufs/fssnap -i /file-system

   For example:

   # /usr/lib/fs/ufs/fssnap -i /usr Snapshot number : 0 Block Device : /dev/fssnap/0 Raw Device : /dev/rfssnap/0 Mount point : /usr Device state : idle Backing store path : /var/tmp/snapshot3 Backing store size : 256 KB Maximum backing store size : Unlimited Snapshot create time : Wed Oct 08 10:38:25 2003 Copy-on-write granularity : 32 KB

3. Back up the UFS snapshot.

   # ufsdump 0ucf /dev/rmt/0 /snapshot-name

   For example:

   # ufsdump 0ucf /dev/rmt/0 /dev/rfssnap/1

4. Verify that the snapshot is backed up.

   For example:

   # ufsrestore tf /dev/rmt/0

How to Create an Incremental Backup of a UFS Snapshot (ufsdump)

Backing up a UFS snapshot incrementally means that only the files that have been modified since the last snapshot are backed up. Use the ufsdump command with the new N option. This option specifies the file system device name to be inserted into the /etc/dumpdates file for tracking incremental dumps.

The following ufsdump command specifies an embedded fssnap command to create an incremental backup of a file system.

Steps

1. Become superuser or assume an equivalent role.

2. Create an incremental backup of a UFS snapshot.

   For example:

   # ufsdump 1ufN /dev/rmt/0 /dev/rdsk/c0t1d0s0 `fssnap -F ufs -o raw,bs= /export/scratch,unlink /dev/rdsk/c0t1d0s0`

   In this example, the -o raw option is used to display the name of the raw device instead of the block device. By using this option, you make it easier to embed the fssnap command in commands (such as the ufsdump command) that require the raw device instead.

3. Verify that the snapshot is backed up.

   # ufsrestore ta /dev/rmt/0

How to Back Up a UFS Snapshot (tar)

If you are using the tar command to back up the snapshot, mount the snapshot before backing it up.

Steps

1. Become superuser or assume an equivalent role.

2. Create a mount point for the snapshot.

   For example:

   # mkdir /backups/home.bkup

3. Mount the snapshot.

   # mount -F ufs -o ro /dev/fssnap/1 /backups/home.bkup

4. Change to the mounted snapshot directory.

   # cd /backups/home.bkup

5. Back up the snapshot with the tar command.

   # tar cvf /dev/rmt/0 .

Restoring Data From a UFS Snapshot Backup

The backup created from the virtual device is essentially just a backup of what the original file system looked like when the snapshot was taken. When you restore a file system from the backup, restore as if you had taken the backup directly from the original file system. Such a restore uses the ufsrestore command. For information on using the ufsrestore command to restore a file or file system, see Chapter 25, Restoring Files and File Systems (Tasks).