Understanding snapshots

What a snapshot is

A snapshot is a frozen, read-only image of the entire Data ONTAP file system that reflects the state of the file system at the time the snapshot was created. Snapshots are your first line of defense for backing up and restoring data.

When snapshots are created

Data ONTAP maintains a configurable snapshot schedule that creates and deletes snapshots automatically. See Creating snapshot schedules. Snapshots can also be created and deleted manually.

Maximum number of snapshots

You can store up to 255 snapshots at one time on each filer volume.

Maximum space snapshots can occupy

You can specify the percentage of disk space that snapshots can occupy. The default setting is 20% of the total (both used and unused) space on the disk. For a full explanation of how snapshots consume disk space, see Understanding snapshot disk consumption.

How snapshots handle file permissions

Snapshot files carry the same permissions and inode numbers as the original files, keeping the integrity of the security system intact. Inodes are data structures that hold information (including permissions information) about files on the filer. There is an inode for each file and a file is uniquely identified by the file system on which it resides and its inode number on that system.

Note
The inode number for a file in a snapshot is the same as the inode number for the corresponding file in the active file system. As a result, some programs on UNIX clients consider the two files to be the same. For example, if you use the GNU diff program to compare the two files, it does not find any differences between them. In some cases, if you try to restore a file from a snapshot, you might see the following error message:
cp:.snapshot/xxx and xxx are identical.
To make sure that the two files have different inode numbers before the copy or comparison, copy one of the files to different name.

What you can do with snapshots

Snapshots enable system administrators to create instantaneous backups from an active filer.

Snapshots enable end-users to

Recover older versions or sets of files that were accidentally changed or deleted
Restore their own files without needing a system administrator to restore files from tape