Surprise! cheap disks cure slow backup


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Just when you thought you knew how to design a backup system - everything changes. A new category of disk array products promises to revolutionize how backups are performed. However, before describing how they work, and what they cost, it's appropriate to explain the problems that they're trying to solve. And it's important to review why backups are performed, and why they are usually sent to tape.

Everyone knows that the only reason we perform backups is to be able to perform recoveries. Recoveries are done for three reasons:

To restore damaged files, file systems or individual systems to their point of failure. Most real-life recoveries aren't done because a data center has been destroyed, or because someone needs a file that was deleted over a year ago. Most recoveries are performed because someone inadvertently deleted an important file, a RAID array was damaged or a database administrator accidentally dropped the wrong table.

To restore a damaged data center to its last available off-site backup. Although recent events have increased the number of people that have actually performed a disaster recovery, most people only worry about having to perform such a recovery. Obviously, the only viable option to recover from a disaster is to store a copy of all backups off-site.

To restore a file, file system or system to an earlier point in time. Sometimes data is corrupted or damaged, and a significant amount of time passes before

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anyone notices that the data is no longer available. Other times, the answer to an important question lies only in a file or database, as it existed 18 months ago. The biggest difficulty with archival restores is storing that much information for that length of time.

For a long time, tape drives and tape libraries have been the only acceptable method to accomplish all three purposes of backup. Optical media's (e.g., MO, CD, DVD) cost per megabyte is usually too high to compete with tape in most environments. Ditto the cost of SCSI disks. Only tape meets all of the criteria mentioned next.

Tape is permanent enough for long-term storage. Because tape is so economical, you can store data that probably won't be needed. And because tape libraries can be expensive, they can be filled again and again with tapes that cost approximately one-tenth the cost of magneto-optical media. Tape can be easily shipped between locations. The most common method of disaster recovery preparation is to create an extra copy of each backup tape and ship it off-site.

The downside of tape
Given the unique capabilities of tape, it's still the best method for accomplishing the disaster recovery and archival purposes of backups. However, these types of restores don't comprise the bulk of restores. Most restores could be better handled by disk - if it were only cheap enough.

Using tape for backups is sometimes a challenging proposition. Many of us have grown use to these challenges, even to the point of pretending they don't exist. For example, tapes are now too fast. Even without compression, the 9840B (19/38 MB/s), AIT-3 (15/30 MB/s), LTO (15/30 MB/s) and Super DLT (11/22 MB/s) drives are all too fast to stream using a single Fast Ethernet connection. In order to stream them, many drives support multiplexing.

This was first published in June 2002

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