Electrical computers and digital processing systems: memory – Storage accessing and control – Control technique
Reexamination Certificate
1999-02-23
2002-05-28
Bragdon, Reginald G. (Department: 2186)
Electrical computers and digital processing systems: memory
Storage accessing and control
Control technique
C711S162000, C707S793000, C707S793000, C707S793000, C380S280000, C380S283000
Reexamination Certificate
active
06397307
ABSTRACT:
BACKGROUND OF THE INVENTION
1. The Field of the Invention
The present invention relates to the protection of computer data, and more particularly to a system and method for mirroring and archiving data of one mass storage to another mass storage.
2. The Prior State of the Art
There is little question that computers have radically changed the way that businesses collect, manage, and utilize information. Computers have become an integral part of most business operations, and in some instances have become such an integral part of a business that when the computers cease to function, business operations cannot be conducted. Banks, insurance companies, brokerage firms, financial service providers, and a variety of other businesses rely on computer networks to store, manipulate, and display information that is constantly subject to change. The success or failure of an important transaction may turn on the availability of information which is both accurate and current. In certain cases, the credibility of the service provider, or its very existence, depends on the reliability of the information maintained on a computer network. Accordingly, businesses worldwide recognize the commercial value of their data and are seeking reliable, cost-effective ways to protect the information stored on their computer networks. In the United States, federal banking regulations also require that banks take steps to protect critical data.
Critical data may be threatened by natural disasters, by acts of terrorism, or by more mundane events such as computer hardware and/or software failures. Although these threats differ in many respects, they all tend to be limited in their geographic extent. Thus, many approaches to protecting data involve creating a copy of the data and placing that copy at a safe geographic distance from the original source of the data. Geographic separation may be an important part of data protection, but does not alone suffice to fully protect all data.
Often the process of creating a copy of the data is referred to as backing up the data or creating a backup copy of the data. When creating a backup copy of data stored on a computer or a computer network, several important factors must be considered. First, a backup copy of data must be logically consistent. A logically consistent backup copy contains no logical inconsistencies, such as data files that are corrupt or terminated improperly. Second, a backup copy of data must be current enough to avoid data staleness. The time between backups, which largely determines the staleness of the backup copy, must be sufficiently short so the data on the backup is still useful should it be needed. For certain applications, such as networks that store financial transactions, backups a week old may be useless and much more frequent backups are needed. How frequent backup copies can be made is a function of many factors such as whether the backup can be made during normal business operations, the time it takes to make a backup copy, and so forth.
In order to create a backup copy of the data, several approaches have been taken. Each of the approaches has certain advantages and disadvantages. Perhaps the simplest approach to creating a backup copy of critical data is to copy the critical data from a mass storage system, such as the magnetic storage system utilized by a computer network, to a second archival mass storage device. The second archival mass storage device is often a storage device designed to store large amounts of data at the expense of immediate access to the data. One type of archival storage commonly used is magnetic tape. In these backup systems, data is copied from the mass storage system to one or more magnetic tapes. The magnetic tapes are then stored either locally or at a remote site in case problems arise with the main mass storage system. If problems arise with the mass main storage system, then data may be copied from the magnetic tape back to either the same or a different mass storage system.
Although using magnetic tape or other archival storage as a means to guard against data loss has the advantage of being relatively simple and inexpensive, it also has severe limitations. One such limitation is related to how such backups are created. When data is copied from a mass storage system to a backup tape, the copy process generally copies the data one file at a time. In other words, a file is copied from the mass storage system onto the tape. After the copy is complete, another file is copied from the mass storage system to the tape. The process is repeated until all files have been copied.
In order to ensure the integrity of data being stored on the tape, care must be taken to keep the file from changing while the backup is being made. A simple example will illustrate this point. Suppose a file stores the account balances of all banking customers. If the account balances were allowed to change during the time the file is being backed up, it may be possible to leave a file in a logically inconsistent state. For example, if one account balance was backed up, and immediately after the account was backed up the account balance was debited $100.00, and if that same $100.00 was credited to a second account, then a situation may arise where the same $100.00 is credited to two different accounts.
In order to prevent such a situation from occurring, the data in a file must not change while the backup copy is made. A simple way to prevent data from changing is to prevent all access to the file during the backup procedure. In such a scheme, access to the files is cut off while the file is backed up. This approach is used by many networks where access to the mass storage system can be terminated after the close of business. For example, if a business closes at the end of each day and leaves its computer network essentially unused at night, user access to the network can be terminated at night and that time used to perform a backup operation. This, however, limits creation of a backup copy to once per day at off hours and therefore may be insufficient for some operations.
An increasing number of computer networks are used by computer businesses that operate world wide, and hence these networks may be needed twenty-four hours a day, seven days a week. Shutting down such a network for several hours each day to make a tape backup may have a significant adverse affect on the business. For such businesses, creating a backup tape in the traditional manner is simply impractical and unworkable.
In an attempt to accommodate such operations or to increase the frequency of backups, an approach to copying data stored on computer networks known as “data shadowing” is sometimes used. A data shadowing program cycles through all the files in a computer network, or through a selected set of critical files and checks the time stamp of each file. If data has been written to the file since the last time the shadowing program checked the file's status, then a copy of the file is sent to a backup system. The backup system receives the data and stores it on tapes or other media. The shadow data is typically more current than data restored from a tape backup, because at least some information is stored during business hours. However, shadow data may nonetheless be outdated and incorrect. For example, it is not unusual to make a data shadowing program responsible for occur in bursts, with heavy activity in one or two files for a short time, followed by a burst of activity in several other files. Thus, a data shadowing program may spend much of its time checking the status of numerous inactive files while several other files undergo rapid changes. If the system crashes, or becomes otherwise unavailable before the data shadowing program gets around to checking the critical files, data may be lost.
Another problem with data shadowing programs is that they typically do not work for data kept in very large files. Consider a system with a single very large database and several much smaller data files. Assuming that a business's primary information is sto
Bragdon Reginald G.
Legato Systems, Inc.
Workman & Nydegger & Seeley
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