Checkpoint restart

Checkpoint restart is a facility offered by some database management systems (DBMSs) and backup-restore software. Checkpoints are taken in anticipation of the potential need to restart a software process.

Many ordinary batch processes on impersonal computers are time-consuming, as are backup and restore operations. They consist of many units of work. If checkpointing is enabled, checkpoints are initiated at specified intervals, in terms of units of work or of processing time. At each checkpoint, intermediate results and a log recording the process's progress are saved to non-volatile storage. The contents of the program's memory area may also be saved.

The purpose of checkpointing is to minimize the amount of time and effort wasted when a long software process is interrupted by a hardware failure, a software failure, or resource unavailability. With checkpointing, the process can be restarted from the latest checkpoint rather than from the beginning.

Checkpoint Frequency

Checkpoints should occur frequently enough to minimize wasted effort when a restart is necessary but not so frequently as to prolong the process unduly with checkpoint overhead. Optimal checkpoint frequency depends on the mean time between failures (MTBF), among other factors.

Disk Staging

Disk staging is using disks as an additional, temporary stage of backup process before finally storing backup to tape. Backups stay on disk typically for a day or a week, before being copied to tape in a background process and deleted afterwards.

The process of disk staging is controlled by the same software that performs actual backups, which is different from virtual tape library where intermediate disk usage is hidden from main backup software. Both techniques are known as D2D2T (disk-to-disk-to-tape).

Data is restored from disk if possible. But if the data exists only on tape it is restored directly (no backward-staging on restore).

Reasons behind using D2D2T:

• increase performance of small, random-access restores: disk has much faster random access than tape
• increase overall backup/restore performance: although disk and a tape have similar streaming throughput, you can easily scale disk throughput by the means of striping (and tape-striping is a much less established technique)
• increase utilization of tape drives: tape shoe-shining effect is eliminated when staging (note that it may still happen on tape restores)

Synthetic Backups

A synthetic backup is identical to a regular full backup in terms of data, but it is created when data is collected from a previous, older full backup and assembled with subsequent incremental backups. The incremental backup will consist only of changed information. A synthetic backup would be used when time or system requirements do not allow for a full complete backup. The end result of combining a recent full backup archive with incremental backup data is two kinds of files which is merged by a backup application to create the synthetic backup. Benefits to using a synthetic backup include a smaller amount of time needed to perform a backup, and system restore times and costs are reduced. This backup procedure is called "synthetic" because it is not a backup created from original data.

Data Encryption

Data Encryption Standard (DES) is a widely-used method of data encryption using a private (secret) key that was judged so difficult to break by the U.S. government that it was restricted for exportation to other countries. There are 72,000,000,000,000,000 (72 quadrillion) or more possible encryption keys that can be used. For each given message, the key is chosen at random from among this enormous number of keys. Like other private key cryptographic methods, both the sender and the receiver must know and use the same private key.

DES applies a 56-bit key to each 64-bit block of data. The process can run in several

modes and involves 16 rounds or operations. Although this is considered "strong" encryption, many companies use "triple DES", which applies three keys in succession. This is not to say that a DES-encrypted message cannot be "broken." Early in 1997, Rivest-Shamir-Adleman, owners of another encryption approach, offered a $10,000 reward for breaking a DES message. A cooperative effort on the Internet of over 14,000 computer users trying out various keys finally deciphered the message, discovering the key after running through only 18 quadrillion of the 72 quadrillion possible keys! Few messages sent today with DES encryption are likely to be subject to this kind of code-breaking effort.

DES originated at IBM in 1977 and was adopted by the U.S. Department of Defense. It is specified in the ANSI X3.92 and X3.106 standards and in the Federal FIPS 46 and 81 standards. Concerned that the encryption algorithm could be used by unfriendly governments, the U.S. government has prevented export of the encryption software. However, free versions of the software are widely available on bulletin board services and Web sites. Since there is some concern that the encryption algorithm will remain relatively unbreakable, NIST has indicated DES will not be recertified as a standard and submissions for its replacement are being accepted. The next standard will be known as the Advanced Encryption Standard (AES).