Tip

Resource guide to snapshots (or point-in-time copies)

Michele Hope, Senior Site Editor

Backups for networked storage have gotten a boost in recent years in the form of services and software that allow you to take rapid "snapshots" of your data at a given point in time. Many systems administrators today rely heavily on snapshot technology or third-party copy vendors to perform this function routinely of their most critical data.

There's some confusion among users, however, about the different types of snapshots available in the form of built in hardware-based, or third-party copy-related functionality. To confuse the matter further, vendors often coin their own derivative name for the snapshot function.

Our experts have been responding to reader questions for months on this issue and have provided some useful background information on where -- and when -- the different types of copy or snapshot features are most appropriate. Here, we present their most useful advice.


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Table of Contents

  Database snapshots via host-based or firmware-based software?
  Differences between hardware-based and software-based metadata snapshots
  Differences between image and snapshot
  Help finding low-cost snapshot products
  How and where to use snapshot technology
  NetApp's SnapShot/SnapRestore features
  SAN School, Lesson 11: Using point-in-time copies
  Snapshots in SQL Server 2000
  Three types of snapshot technologies
  The truth about synchronous replication
  Use snapshot for open files


Database snapshots via host-based or firmware-based software?
When is it appropriate to take snapshots of your data via host-based software solutions, such as Veritas' Virtual Replicator or Sun's Instant Image? According to our SAN expert, you also have the option to take snapshots from some firmware-based software performed at the storage subsystem level. Here, you'll learn about the different modes of licensing each type of software, along with the benefits/disadvantages between taking clone copies and metadata snapshots. You'll also learn about the role quiescing your database plays in the snapshot creation process.

Differences between hardware-based and software-based metadata snapshots
Expert Chris Poelker explains how the HDS 9000 storage arrays use a cascaded snapshot approach where writes to the original volume are given an I/O complete back to the application immediately. He also describes how this approach differs from other software-based snapshot functionality.

Differences between image and snapshot
Backup expert W. Curtis Preston contrasts performing an image backup via Unix with the process of producing snapshots. Do you know when a snapshot is considered a true mirror? Learn when Preston thinks this is the case in this expert answer.

Help finding low-cost snapshot products
Expert Chris Poelker evaluates PowerQuest V2i for use as a low-cost snapshot product in an enterprise-class SAN. He also describes other implementations readers may want to look at for third-party snapshot products.

How and where to use snapshot technology
Is there a difference between the terms: snapshot, clone and snapclone? According to expert Christopher Poelker, these are all different terms coined by various vendors, often to mean the same thing. Learn here what Chris says are the two basic methods for creating snapshots of your data.

NetApp's SnapShot/SnapRestore features
NAS expert Randy Kerns explains how Network Appliance's NAS filers currently handle blocks of data written via the products' SnapShot features and how these may compare to other products on the market.

SAN School, Lesson 11: Using point-in-time copies
Lesson 11 in our short SAN learning-related webcasts covers the use of point-in-time copies, which are also referred to as snapshots. Here, you'll learn straight from the coauthor of the book, "Storage area networks for Dummies," what point-in-time copy is and what you need to know about creating snapshots. You'll also learn how to figure out space requirements and the types of software-based copy solutions available. (Note: This lesson may not yet be available for viewing.)

Snapshots in SQL Server 2000
Learn how to create snapshots of SQL Server 2000 data via this technical tip.

Three types of snapshot technologies
Storage networking expert Marc Farley claims that snapshot technology comes in three basic flavors: file system-based, subsystem-based, and virtualization-based. Learn the differences of each type in this expert answer.

The truth about synchronous replication
Learn how snapshots can be used successfully in an asynchronous replication scheme to ensure recovery of database data from a known "good" point. Contributor Geoff Barrall, CTO of BlueArc Corporation, also describes here the merits of synchronous vs. asynchronous replication strategies.

Use snapshot for open files
In this tip, you'll learn why Veritas says open files are responsible for 97 percent of incomplete backups. You'll also learn how snapshot technology can avoid this common pitfall.

This was first published in December 2003

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