Disk-to-disk-to-tape (D2D2T) is an approach to computer storage backup in which data is initially copied to backup storage on a disk storage system (Storage Array) and then periodically copied again to a tape storage system (or an optical storage system).

Traditionally, many businesses have done backup directly to relatively inexpensive tape systems. However, for many computer applications, it's important to have data immediately ready to be restored from a secondary disk if and when the data on the primary disk becomes inaccessible (for example, if the server fails). The time to restore data from tape would be considered unacceptable. On the other hand, tape is a more economical alternative for long-term storage. Because it's also more portable, tape is often used for off-site backup and restoration in case of a disaster.

Disk-to-disk-to-tape is often used as part of a storage virtualisation system. In such a system, data that is more likely to require restoration from a backup device may be kept on an on-site or off-site disk storage system; data, such as e-mail, that has less value over time, may be migrated on a set schedule to tape (or all email messages archived using appliance based technology). The storage administrator can express a company's needs in terms of storage policies rather than in terms of the physical devices to be used.

There are several good reasons to add a disk stage to your backup architecture. Ask most storage professionals about disk to disk and you'll hear their fantasies of stunning raw backup performance.

• Reduced backup times during the disk stage, but this probably won't be caused by faster backups.

e.g. Today's SDLT (Super Digital Linear Tape) and LTO (Linear Tape Open) tape drives are capable of backup rates faster than 2 GB per minute--quicker than most small and midsize business servers can shoot the data to them. So disk backups aren't actually faster than today's tape drives. An LTO3 drive will clock in at 1.2 GB per minute to 1.8 GB per minute, as will an optimised disk system.

• Disk backups save time because you can run more than one job at once.

e.g. With most affordable tape-backup programs, you can back up only one data stream at a time. If you have 10 servers that each need two hours to back up, and only one tape drive, a full backup will require a hefty 20 hours--and a baby sitter to swap tapes. Most servers, especially Exchange servers, can't send data that fast, so the backup server waits. Enterprise backup programs, (Legato, Veritas, BrightStor, Commvault, etc) intersperse data on tape at the cost of slowing down restores because the system must skip over other data. In contrast, because a disk is a random-access medium, you could back up four or five servers at a time before saturating the disk I/O channel, turning a 20-hour backup into a five-hour process.

D2D2T Software Features Click to Enlarge In today's tape-only world, most administrators back up to prepare for a catastrophic server failure. They make full backups nightly or full and differential backups, making many copies of the same data to avoid having to locate and mount each series of incremental backup tapes if they have to rebuild a server.

• Restore single Files Not a whole Drive/Server

e.g. Tapes are fine if you're restoring an entire drive, but let's face it: A typical restore is a response to a frantic phone call from a user who just accidentally deleted or overwrote a critical file or realised a file has become corrupt. Pity the poor administrator who must figure out which tape contains the appropriate data, load it into the tape library or drive and start the restore job, which may take an hour or more before the backup software locates the file on the sequential access medium.

Add a disk stage to the backup system and you can stop all those duplicate backups by using incremental backups. Because all the incremental backups are online in the disk data vault, a full server rebuild from a full backup and several incrementals isn't a huge chore. As a result, you can back up less data and, therefore, take less time to run your backups. When you receive the panicked call for a file restore, the data will be online, and you can restore the panicked caller's file in a couple of minutes--as opposed to a couple of hours.

D2D2T means more than backing up your servers to a set of files and then backing up the files to a tape. You could do that with ntbackup, but you'd have to restore from tape to backup server disk and then to the source server. The tape from a D2D2T backup should have the same data as if you'd backed up directly to tape, so backup applications should provide a data-copy function. You also can create a synthetic full backup by using your application's database of previously stored files to build a backup set--on disk or tape--that contains the same data as a full backup. If your application supports synthetic full backups, you can run a full backup from your server just once and the much smaller and faster incremental backups from that point forward.

By employing disks for day-to-day backups, you can reserve tape for those applications where portability, long shelf life, zero offline power consumption and relatively low cost come into play, such as for off-site backups and long-term archival storage.

Advantages of D2D2T:

• Quick backups & restores will reduce network bandwidth issues

• Reduced backup window concerns

• Consolidated storage

• Portability of tape provides security against fire, theft, viruses, & natural disaster

• Tape provides a cost effective solution for long term archive

Disadvantages of D2D2T:

• More complicated to setup

• Additional cost (Depending on Straight D2D or Virtual Tape Library Setup)

• More points of failure

To findout more about Disk-to-Disk-to-Tape Backup Products and Solutions contact AustSTOR:

Tel: (toll free Australia) 1300 134 795
Int: (+612) 4324 0522
email: sales@auststor.com.au

Related Links:

Technology Corner - D2D

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