Disk to Disk to Tape (D2D2T) Data Protection Architecture

With the high capacity and lower cost offered by SATA and Fibre Channel ATA (FATA) disk technologies, many customers are now considering implementing backup to low cost disk arrays before backup to tape.

The use of secondary disk arrays for backup is best suited to environments where:

•The business dictates rapid single file restore capabilities (seconds to minutes). It is generally quicker to restore from a secondary disk subsystem than it is from tape (minutes to hours). However, the secondary storage array is not infinite in capacity and only the most recent backups may still reside on the disk array. In addition, disk-to-disk backup is no substitute for offsite media.

•The hosts can only supply data at a relatively modest rate (10–20 MB/s). The backup image can be gradually produced on the secondary array without having to interleave multiple streams to tape and without the need for a large powerful dedicated backup server.

•Small file backup to tape has always been a performance limiter. With backup to disk, a complete backup image of small files can be constructed and then passed to tape at much higher transfer rates than if the small files were transferred directly to tape.

Table 2. Comparing SATA and SCSI Disks

Feature

SATA Disks

SCSI Disks

 

 

 

Mean time to failure (MTTF)

500,000 hrs @ 20% duty cycle

1,200,000 hrs @ 100% duty cycle

Burst transfer rate

150 MB/s

320 MB/s

RPM

7.2K

10K or 15K

Queuing

Non-Tagged

Tagged

 

Serial execution

Optimized seeks

 

 

Better performance

Capacity

250 GB

146 GB

Warranty

1 year

3 years

 

 

 

While the SCSI disk MTTF of 1,200,000 looks high, this figure decreases when there are many disk drives bound together in an array. By comparison, Ultrium 460 tape drives have a MTTF of 250,000 at 100% duty cycle but are used in smaller volumes within an automated tape library. Therefore, do not assume that disk is automatically more reliable than tape.

The key point is D2D2T has its place and with proper administration can improve the data protection process but it is not a replacement for tape. Tape is still the foundation of a robust data protection strategy.

HP OpenView Storage Data Protector 5.5 provides a comprehensive implementation of D2D2T called “Advanced Backup to Disk.” The following example shows a scenario with a single central backup server and multiple network connected clients:

•The first step is to backup clients over the network to a central backup server and its staging area.

•The second step is to copy the consolidated backups from the staging area to tape. Typically the central backup and restore environment is designed to provide high speed backup from disk to tape.

•If a restore is required, data can be accessed either from disk (if still available) or from tape over the network.

10