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 initially 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.
Caution - 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.
You can also specify a directory for the backing-store path, which means a
backing store file is created in the directory specified. For example, if
/var/tmp is specified for the backing-store path, the following backing-store file is created.
/var/tmp/snapshot0
If you created one large root (/) file system instead of creating separate
file systems for /export/home, /usr, and so on, you will be unable
to create a snapshot of those separate file systems. For example, this system
does not have a separate file system for /usr as indicated under the
Mounted on column:
# df -k /usr
Filesystem kbytes used avail capacity Mounted on
/dev/dsk/c0t0d0s0 3618177 2190002 1391994 62% /
If you attempt to create a snapshot for the /usr file system, you
will see a message similar to the following:
# fssnap -F ufs -o bs=/snaps/usr.back.file /usr
snapshot error: Invalid backing file path
This message indicates that you cannot have the backing store file on the
same file system as the file system being snapped, which is the
case for the /usr file system, in this example.
For more information, see the fssnap_ufs(1M) man page.
Creating a Multiterabyte UFS Snapshot
Creating a multiterabyte UFS snapshot is identical to creating a snapshot for a
smaller UFS file system. The only difference is that multiple backing store files
are created for each 512 Gbytes of file system space.
Keep the following key points in mind when creating a snapshot for
a file system that is larger than 512 Gbytes:
Multiple backing store files are created.
If you specify a backing store file name when the snapshot is created, then the subsequent backing store file names will be interated based on the file name that you specify. The subsequent backing-store files will have the same name, but with the suffixes .2, .3, and so on.
If you only specify a backing store file destination (or directory) and not a backing store file name, then multiple backing store file names will be created and iterated with the suffixes .2, .3, and so on.
The fssnap -i command only reports the first backing store file name even if multiple backing store files have been created. However, the reported backing-store length is the combined sizes of all the backing store files for the snapshot.
Note - 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.
After you have backed up the snapshot or you would just like to remove the snapshot, you will have to remove the backing store files manually if you did not use the unlink option when the snapshot was created.
For an example of creating a snapshot for a file system that
is larger than 512 Gbytes, see Example 26-2.
For more information, see fssnap_ufs(1M).
How to Create a UFS Snapshot
- Become superuser or assume an equivalent role.
- Make sure that the file system has enough disk space for the backing-store
file.
# df -k
- Make sure that a backing-store file of the same name and location does
not already exist.
# ls /backing-store-file
- 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.
- Verify that the snapshot has been created.
# /usr/lib/fs/ufs/fssnap -i /file-system
Example 26-1 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 /usr
/dev/fssnap/1
Example 26-2 Creating a Multiterabyte UFS Snapshot
The following example shows how to create a snapshot of a 1.6
Tbyte UFS file system.
# fssnap -F ufs -o bs=/var/tmp /data2
/dev/fssnap/0
# /usr/lib/fs/ufs/fssnap -i
Snapshot number : 0
Block Device : /dev/fssnap/0
Raw Device : /dev/rfssnap/0
Mount point : /data2
Device state : idle
Backing store path : /var/tmp/snapshot0
Backing store size : 0 KB
Maximum backing store size : Unlimited
Snapshot create time : Fri Sep 10 13:13:02 2004
Copy-on-write granularity : 32 KB
# ls /var/tmp
snapshot0 snapshot0.2 snapshot0.3 snapshot0.4
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 file system snapshot. If you don't specify a file system, you
see information about all of the current UFS snapshots and their corresponding virtual
devices.
Note - Use the UFS file system-specific fssnap command to view the extended snapshot information
as shown in the following examples.
- Become superuser or assume an equivalent role.
- 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 : /export/home
Device state : idle
Backing store path : /var/tmp/home.snap0
Backing store size : 0 KB
Maximum backing store size : Unlimited
Snapshot create time : Thu Jul 01 14:50:38 2004
Copy-on-write granularity : 32 KB
- Display detailed information about a specific snapshot.
For example:
# /usr/lib/fs/ufs/fssnap -i /export
Snapshot number : 1
Block Device : /dev/fssnap/1
Raw Device : /dev/rfssnap/1
Mount point : /export
Device state : idle
Backing store path : /var/tmp/export.snap0
Backing store size : 0 KB
Maximum backing store size : Unlimited
Snapshot create time : Thu Jul 01 15:03:22 2004
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 manually delete the backing-store file.
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 manually delete
the file.
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 specify the path
of the file system that contains the UFS snapshot.
- Become superuser or assume an equivalent role.
- Identify the snapshot to be deleted.
# /usr/lib/fs/ufs/fssnap -i
- Delete the snapshot.
# fssnap -d /file-system
Deleted snapshot 1.
- 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 26-3 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 /export/home
1 /export
# fssnap -d /usr
Deleted snapshot 1.
# rm /var/tmp/export.snap0
