Backup storage appliances come of age - Security

Computer Technology Review, Nov, 2003 by Brian Biles

Data is one of the corporation's most important assets, but it's also one of the corporation's biggest headaches. Data needs to be fully protected in the face of growing data volumes, the demand for shorter restore windows, and reduced storage expenditures. Believing that "two out of three ain't bad," many companies have sacrificed short restore windows for tape's large capacity and economical price point. Most know that backup to disk can enable much faster restores with higher reliability and easier administration, but in the past the cost has been too high.

Traditional backups have relied on tape because it is cost effective for fast backups and getting data offsite, but it is slow and unreliable for restores--the main reason backups happen in the first place. Other types of data protection do rely heavily on disk (replication, mirroring and snapshots are prime examples) but IT reserves these more expensive methods for critical data with high availability and rock-solid recovery requirements. Disk is also ideal for data warehouses and repositories, but having to house multiple backup versions makes heavy demands on disk-based backup. On top of cost issues, getting an integrated backup function to work across multiple applications, hosts and storage targets is extremely challenging.

More and more companies have been looking at backup appliances to let them use disk-based backup at a reduced price point, but these systems have lacked advanced data protection features. The challenge for backup product makers is to provide a disk-based long-term storage system that supports multiple backup applications, multiple versions of backups, allows restore to use disk's random access capability, has a comparable price point to tape, and ensures reliable and verifiable recoveries.

Backup and Recovery Requirements

Many disk manufacturers have triumphantly announced the death of tape, but tape sales remain healthy. It's easy to see why: tape technology advances have included better robotics, increasing capacity and performance on drives and media, and the ability to store multiple copies or versions at a low cost and on removable media. However, manual tape handling is awkward and prone to error, tape needs careful tuning to optimize streaming, and backup quality can be hard to determine until you go to recover--at which point you really hope the quality is there.

One of the main technical problems with tape drives is that they must be tuned to avoid what is called the shoeshine effect--starting, stopping and repositioning the tape. To avoid this, it's much more efficient to stream tape drives by sending several concurrent backups. This process, called multiplexing, does help backup performance by minimizing the shoeshine effect. However, it takes extra time to read images and handle multiple incoming backup sources.

Disk storage counters these disadvantages by avoiding the shoeshine effect, handling incremental backups better, making off-site backup easier and more efficient, and providing more reliable recovery:

* Avoiding the shoeshine effect. Disk arrays do not need a steady stream of data, so there is no shoeshine effect even for small incremental backups.

* Simplifying and accelerating the backup process. Storage administrators like incremental backups because they help shrink backup windows, but tape restores are simpler and more effective with full backups. Disk-based backup systems allow administrators to schedule more incremental backups without worrying about performance penalties or risking restores.

* Making off-site backup copies easier and more efficient. Since it isn't multiplexing backup data from multiple backup clients on one tape, disk-to-tape copying efficiently organizes backup data by client, which speeds up the recovery process. Disk-to-tape copying also provides more flexibility than tape: tape-to-tape copying does not allow other backup or restore operations, but disk allows simultaneous access.

* Superior recovery. Disk-specific technologies like RAID make disk a more reliable medium than tape. One bad tape can cause an entire restore operation to fail, but RAID protection allows a restore to complete successfully even with a failed disk.

* Efficient single-file recoveries. According to analyst firm Strategic Research, 87% of all restores are single-file recoveries, not full-system recoveries. Disk is a random access medium, and is ideal for single-file recovery.

In spite of these advantages, people are still backing up to tape. So what's the problem? Disk has lagged behind tape as a long-term storage medium because of cost. Consider the price of storing four weeks' worth of full and daily incremental backups. Here's the math: If you assume that incremental backup sizes are about 5% of the original data set, over a month's time the backups will consume five times the original data size. Given this capacity, using primary disk subsystems for backup storage is cost prohibitive. Less expensive ATA-based disk arrays are cheaper, but at $10-$20/GB they're still much more expensive than $1-$5/GB for tape-based backups.