The optimal backup solution: it's now within your reach

Computer Technology Review, Oct, 2004 by Jim McKinstry

A very common problem in the data center is backing up data in a timely manner. IT managers struggle with the constantly shrinking backup window while their data continues to grow at astronomical rates. In today's world, there is no need to suffer with clunky, multi-hour backup windows; SAN-enabled backups can dramatically reduce or completely eliminate the amount of time needed to backup data.

Traditionally, each server that required backup had a tape drive or small library attached. This solution allowed for fast backups but was expensive to deploy and complex to manage. Over time, companies deployed large backup servers with large libraries attached and performed backups over the LAN. These solutions are easily able to grow as more data is brought online. Unfortunately, with growth they become more expensive and more complex to manage.

Enter the SAN

Servers attached to a SAN can quickly (and cost effectively) take advantage of the SAN to backup data. By attaching backup servers to the SAN and enabling "IP-over-Fibre" for them, the SAN-attached servers will allow backup traffic that used to travel over the LAN to travel over the SAN using IP. Changes to the backup server will be needed but are minor. For companies that are still using 10/100 networks for backups, the performance increase will be dramatic. Companies using Gig-E for backups may see a performance increase, but will see less traffic flowing over the production network during backups.

SAN and Tape

Adding tape libraries to a SAN allows users to address backup issues in a variety of ways. For many companies, it is difficult to efficiently make use of their tape libraries. Some libraries are constantly busy while others are idle. Adding the backup servers and tape libraries to a SAN gives administrators the flexibility to configure their backups to use the resources efficiently. Many backup servers can utilize large libraries attached to a SAN instead of dedicating a large (expensive) backup server directly to a large library. Also, many smaller libraries can easily be consolidated into a large, SAN-attached library.

For many backup environments, the bottleneck is the network. Once a library is attached to a SAN, every server attached to the SAN has access to it. Most backup software will allow a server attached to the SAN to back itself up directly to the SAN-attached library. These servers will backup at tape speeds since they will no longer send their data over the network. Since servers attached to the SAN usually have a large amount of data, their backups usually occupy a large amount of the backup servers' time. By having the application servers back themselves up, not only will their backups finish quicker, resources on the backup servers will be free to backup the rest of the environment faster.

Disk-to-Disk-to-Tape

The next logical step in leveraging the SAN to solve the backup problems is to consider backing up production data to SAN-attached disk. This allows backups to finish very quickly, usually faster than backing up to tape and more importantly, data recovery is very fast. The biggest drawback of disk-based backup is that there is no tape that can be stored offsite. This is solved by copying the data to tape after the backups are complete. Besides the raw performance increases it provides, disk is also faster than tape because the disk-based system does not have to physically find a tape in the library, load it, and advance the tape to the data. With disk-based backup systems, "tapes" are loaded instantly and access to data, anywhere on the "tape", is instantaneous. There are many ways to implement a disk-based backup solution.

Virtual Libraries

Vendors are starting to introduce libraries that use inexpensive ATA disks with a virtualization engine front end, which emulates a tape library to the backup software running on the backup server. This type of disk-based library typically emulates a library with many tape drives and tape slots. There is nothing special that needs to be done to the backup software; it simply sees another multidrive/multi-slot library that it can use. Attaching one of these virtual libraries to a SAN allows them to be used in the same manner as the SAN-attached tape libraries described above.

Some backup software vendors have an option to allow for backing up directly to disk. With the same benefits of the hardware-based systems, the software is also more cost effective and has the flexibility to use any available type of disk storage. While the hardware implementations limit users to the amount of backup data that can be stored on a unit, the software version lets them use as much disk as they'd like, enabling them to store huge amounts of backup data near line. Using SAN-attached disk allows for virtually unlimited capacity that can be allocated for backups.

Snapshots

The ultimate in backup technology is the snapshot. A snapshot is, after all, just a copy of data, which is exactly what a backup is. Snapshots are considered a disk-based backup solution because they are stored on disk. During a backup, the applications on the servers being backed up are either in degraded mode (e.g., a database is put into hot backup mode) or are shut down (called a cold backup). With a library (tape or disk), the applications are degraded or down for a large amount of time, frequently for several hours. With snapshot technology, applications are impacted only for seconds or minutes. Usually, it takes longer for the application to shut down or a database to enter hot-backup mode than it takes to perform the snapshot. Snapshots are powerful because a system can be backed up in seconds, then archived to tape at any time. With a SAN-attached tape library, backup performance would increase by allowing SAN-attached application servers to back themselves up directly to the library. With a snapshot, it can be mounted to a SAN-attached backup server and archived to tape with virtually no overhead or impact to the application server.