White Paper

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
Serial execution
Tagged
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.
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