Tape and Backup Issues In Storage Area Networks - Technology Information

Computer Technology Review, April, 2001 by Robert A. Jackson, Erik Johannessen, Rich Mikulak, Matt Reller, Bill Peldzus, Jeff White

Authors' note: This article addresses tape and backup issues in storage networks, with a focus on case studies and other consulting and testing experiences from Imation's Storage Network Solutions Lab, in Oakdale, MN.

As the adoption rate of storage networks continues to grow quickly, customers face fundamental challenges with the architecture and design of the SAN infrastructure. Moreover, there are a myriad of migration challenges--from understanding and mapping what is in place today (typically SCSI-based systems) to anticipating what will be installed tomorrow (based upon the latest industry reports, most commonly Fibre Channel systems).

Disk and tape are two of the most widespread storage devices in an open systems environment. Between these two devices, we've found that more storage networking connectivity issues are centered around the tape environment. This is of considerable concern as it can be argued that the biggest "killer app" that can benefit from storage networking is the backup and restore application--an application relying heavily on the interaction between disk and tape.

Backup and restore applications are the thorn in the side of nearly every IT director. It is analogous to buying home or auto insurance--you hate to pay the monthly premium, but if you don't have it when you need it, the results can be disastrous. As corporations' storage requirements continue to double or triple each year, a backup and restore application designed to be extremely cost-effective three years ago is now bursting at the seams trying to accommodate these new storage volumes. If that backup infrastructure is primarily LAN-based, potentially over a shared 10Mbit Ethernet, it is not uncommon to hear stories of daily backup windows ranging from 13-23 hours every day and weekly backups taking the entire weekend. Backup and restore applications are well positioned to take advantage of every business and technical benefit that storage networks can provide. Because tape is primarily the key media in these applications, we continue to uncover hidden challenges of configuring tape drives in storage network s as we test and stress these applications and environments in the lab.

Use of Disk And Tape

Before we review some lab experiences, we'd like to note the subtle differences between disk and tape and the way they have been used in IT environments in the past and today.

Regardless of the connection method, disk gained the primary attention of high availability experts. Disk, with spinning parts and moving heads, was tagged as one of the first devices with the highest potential to fail over a specified period of time (also referred to as Mean Time Between Failures--MTBF). That failure potential drove the advent of RAID levels, RAID disk subsystems, and more robust drivers developed with a built-in level of recoverability in the event a hard or soft error occurs, which is often expected.

While tape also has moving parts and heads, a failure of a tape drive or tape media did not get the same attention as a failed, unprotected disk drive. Applications and data resided on online disk, and if it failed your application was down. Backup data resided on offline or nearline tape, and if it failed you simply re-ran the backup job.

In today's environment most, if not all, of a company's online disk is protected in some RAID fashion. A disk failure is now an afterthought, with customer-serviceable hot-swap disk drives, global hot spares, and algorithms to predict when a disk is going to fail before it actually fails so that it can be replaced ahead of time.

However, in the backup and restore environment, a failed I/O still equates to a failed backup job. Although this job can be automatically retried, it must start at the beginning. With the volume of data being backed up today, a single, failed backup job could mean not meeting the nightly backup window. There also are protection schemes for tape, such as RAIT (Redundant Array of Independent Tape) and RAIL (Redundant Array of Independent (Automated) Libraries), but these solutions are expensive and therefore uncommon.

With the introduction of tape sharing in storage networks, the possibility of failed I/O has increased, and with it the potential for missed backup and/or restore windows has increased as well. As customers want to maximize the value of current backup assets, most commonly SCSI-based tape drives, they need more equipment between the disk storage and the tape storage such as hubs, switches, bridge/routers, etc., to enable advanced features and functionality. All of this further increases the complexity and the potential for problems.

The IT Director's back is to the wall when looking at backup and restore. Should he or she invest in old or new technology to accommodate this explosive data growth? For reasons that drive migration to new tape technologies, customers are also migrating to a newer backup/restore storage networking architecture. While we feel that this is the right decision for the right reasons, we will discuss some of the more interesting technical challenges specific to backup and restore applications in storage networks.