N series Snapshot: A Technical Discussion
An IBM Redpaper publication
Note: This is publication is now archived. For reference only.
This IBM Redpaper discusses the N series Snapshot feature. Snapshot is a standard feature of the Data ONTAP operating system. It enables online backups to be maintained, thus providing near-instantaneous access to previous versions of data without requiring complete, separate copies or resorting to offline backups. Snapshots can be scheduled by an administrator, and you can keep up to 255 Snapshots online at any one time. The paper provides UNIX and Windows examples of creating Snapshots. Snapshot technology makes extremely efficient use of storage by storing only block-level changes between each successive Snapshot. In this way, it can be simply thought of as being similar to any modern source code control system that maintains only the changes made to the original source code in order to minimize space and maximize recoverability. This analogy actually extends further, and we explore it later in this document. Because the Snapshot process is automatic and virtually instantaneous, backups are significantly faster and simpler. Snapshots can also be coordinated with outside applications to ensure highly consistent data states (as viewed from the application) prior to performing Snapshot and other backup procedures. For example, flushing data from a production database prior to Snapshot creation is a generally recognized best practice. The primary focus of this paper is on the algorithms and data structures that Write Anywhere File Layout (WAFL) uses to implement Snapshots, which are read-only clones of the active file system. WAFL uses a technique to minimize the disk space that Snapshots consume. This paper also describes how WAFL uses Snapshots to eliminate the need for file system consistency checking after an unclean shutdown, and gives examples of how to set up a Snapshot using the FilerView, Data ONTAP CLI, SnapDrive for Microsoft Windows and SnapDrive for Linux and UNIX. The WAFL primary distinguishing characteristic is Snapshots, which are read-only copies of the entire file system. WAFL creates and deletes Snapshots automatically at prescheduled times, and it keeps up to 255 Snapshots online at once to provide easy access to old versions of files.