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|System Administration Guide: Devices and File Systems Oracle Solaris 10 8/11 Information Library|
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.
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 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 GB of file system space.
Keep the following key points in mind when creating a snapshot for a file system that is larger than 512 GB:
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 GB, see Example 24-2.
For more information, see fssnap_ufs(1M).
# df -k
# ls /backing-store-file
# 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.
# /usr/lib/fs/ufs/fssnap -i /file-system
Example 24-1 Creating a UFS Snapshot
# 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 MB.
# fssnap -F ufs -o maxsize=500m,bs=/scratch/usr.back.file /usr /dev/fssnap/1
Example 24-2 Creating a Multiterabyte UFS Snapshot
The following example shows how to create a snapshot of a 1.6 TB UFS file system.
# fssnap -F ufs -o bs=/var/tmp /datab /dev/fssnap/2 # /usr/lib/fs/ufs/fssnap -i /datab Snapshot number : 2 Block Device : /dev/fssnap/2 Raw Device : /dev/rfssnap/2 Mount point : /datab Device state : idle Backing store path : /var/tmp/snapshot3 Backing store size : 0 KB Maximum backing store size : Unlimited Snapshot create time : Mon Jul 12 10:37:50 2010 Copy-on-write granularity : 32 KB
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.
# /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/snapshot0 Backing store size : 0 KB Maximum backing store size : Unlimited Snapshot create time : Mon Jul 12 10:37:50 2010 Copy-on-write granularity : 32 KB
# /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/snapshot1 Backing store size : 0 KB Maximum backing store size : Unlimited Snapshot create time : Mon Jul 12 10:37:50 2010 Copy-on-write granularity : 32 KB
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.
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.
# /usr/lib/fs/ufs/fssnap -i
# fssnap -d /file-system Deleted snapshot 1.
# rm /file-system/backing-store-file
Example 24-3 Deleting a UFS Snapshot
# fssnap -i 0 /export/home 1 /export # fssnap -d /export Deleted snapshot 1. # rm /var/tmp/snapshot1