Different Levels of Data Protection
Backup and Restore, describes the role of LOGDB and TRANSDB, when used in conjunction with backup and restore, in protecting data against loss.
Databank shadowing provides an even higher level of protection. Listed below are the different ways in which data can be protected from loss (from the least amount of protection to the highest).
All Databanks on One Disk and No Logging
If a databank is lost with this level of protection, it is only possible to continue operations from the last backup copy (all changes since the last backup was taken are lost).
Databanks can be lost due to accidental deletion, disk crashes (which can destroy all files on a disk), etc. This level of protection is not recommended except for "trash" databanks with unimportant contents.
Logging, with LOGDB and TRANSDB on a Separate Disk from the Data
LOGDB and TRANSDB are vital databanks if the system stops or if any databanks are lost. Because of this, LOGDB and TRANSDB should be stored on separate disks, as shown in the following figure.
Application data should be stored on the TRANSDB disk if it cannot be stored separately.
If a databank is lost, it can be restored to the state it had when it was lost by applying the transactions in LOGDB and TRANSDB to a restored backup copy of the databank.
This may take some time, especially if the databank is large and if there is a lot of transaction information stored in LOGDB.
Caution: If the databank disk and the TRANSDB/LOGDB disk are handled by the same disk controller, a disk controller failure may cause both disks to crash.
This security level gives a high degree of security and is recommended for databanks containing important data used in a system where the delay before the system is restored after a crash is not critical. To assure this high degree of security the backup copies should always be stored on separate removable media (e.g. tapes).
Shadowing, with Shadows on a Separate Disk
Shadows should always be stored on a separate disk from the masters to protect them from a total disk crash that could destroy both the master and shadow databanks.
It is also advisable to use separate disk controllers to assure that a corrupt disk controller does not destroy the disks holding both the masters and the shadows.
If a databank is lost, its shadow can be transformed into a master and the shadow automatically takes over with no loss of data.
Since the shadows are updated after the master, and the operations are saved in TRANSDB until the shadow is updated, it is important to have a good copy of TRANSDB when a shadow is transformed.
To assure this, it is advisable to shadow TRANSDB. It is strongly recommended that TRANSDB and its shadows are stored on separate disks, as shown in the following diagram.
This arrangement gives a high degree of security and is recommended for databanks containing important data used in a system where it is vital to be able to get the system running again quickly after a disk crash.
Shadowing and Logging
Combining shadowing and logging, see Backup and Restore, gives the highest level of data protection.
If logging is not used, the data is not protected if both the master and the shadow disks become corrupted. However, when shadowing is combined with logging (with LOGDB on a third disk) and backups are regularly taken onto separate media (tape, etc.), then data is always protected if any two of the disks crash.
Of course, additional disks can be used, just as long as the databanks that are separated above are not placed onto the same disks. If you only have two disk drives available and all the databanks are shadowed, then logging is of little value. Shadowing of LOGDB will not increase data protection significantly in this configuration.
Different degrees of data security can be used for different databanks, depending on the importance of the data. It is however important that all inter-dependent databanks (because of foreign key relationships, etc.) have the same level of protection. Otherwise logical inconsistencies may result if there is a disk crash.
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