Using snapshots

How Data ONTAP snapshots work in an FC network

Data ONTAP cannot take snapshots of some applications that are implemented on top of file systems, such as NTFS. If you want to take a snapshot of these types of applications, you must first ensure that the files are closed and cannot be modified, and that the application is quiesced, or taken offline, so that the file system caches are committed before the snapshot is taken. The snapshot takes less than one second to complete, at which time the application can resume normal operation.

If the application requires a lot of time to quiesce, this can result in some amount of unavailable time. To avoid this scenario, some applications have a built-in hot backup mode. This allows a snapshot or a backup to occur while the application operates in a degraded mode, with limited performance.

Data ONTAP cannot take snapshots of applications that have the ability to work with raw device partitions. NetApp recommends that you use specialized modules from a backup software vendor tailored for such applications.

If you want to back up raw partitions, it is best to use the hot backup mode for the duration of the backup operation. For more information about backup and recovery of databases using NetApp SAN configurations, see the appropriate Technical Report for the database at http://www.netapp.com/tech_library.

How snapshots are used in the SAN environment

Data ONTAP cannot ensure that the data within a LUN is in a consistent state with regard to the application accessing the data inside the LUN. Therefore, prior to creating a snapshot, you must quiesce the application or file system using the LUN (which flushes the host filer system buffers to disk) so that the snapshot is consistent. For example, you can use batch files and scripts on a host that has administrative access to the filer. You use these scripts to perform the following tasks:

Make the data within the LUN consistent with the application, possibly by quiescing a database, placing the application in hot backup mode, or taking the application offline
Use the rsh or ssh command to create the snapshot on the filer (this only takes a few seconds, regardless of volume size or use)
Return the application to normal operation
Note
On Windows hosts, you can use the Windows Task Scheduler service to execute this script at specified intervals.
In addition, you can use SnapDrive 2.0 or later to save the contents of the host file system buffers to disk and create snapshots. See the SnapDrive Installation and Administration Guide.

The relationship between a LUN and a snapshot

When you take a snapshot of a LUN, it is initially backed by data in the snapshot. After the snapshot is taken, writes to the LUN reside in the active file system. The backing snapshot cannot be deleted without destroying this LUN.

What you can do with a snapshot

After you have a snapshot, you can use it to create a LUN for temporary use as a prototype for testing data or scripts in applications or databases by creating a writable snapshot of a LUN (a backing store LUN) with the lun create -b command. Because the LUN is backed by the snapshot, you cannot delete the snapshot until the backing store LUN is deleted.

If you want to retain the LUN on a permanent basis, or want to delete the snapshot that backs the backing store LUN, you can clone the backing store LUN and then delete the snapshot. LUN clones require the same amount of space as the original LUN, whereas backing store LUNs require the amount of space of the original LUN plus the delta in the snapshot.

If you want to restore the LUN from a snapshot, you can use SnapRestore, but it will not have any updates to the data since the snapshot was taken.

What snapshots require

In Data ONTAP 6.5, LUNs are space reserved by default. This means that enough space is reserved so that writes to the LUNs are guaranteed. The more space that is reserved, the less free space is available. If free space within the volume is below a certain threshold, snapshots cannot be taken. For information about how to manage available space, see Monitoring disk space.