 |
Claims  |
|
|
We claim:
1. A method for storing information in a clinet-server environment having a
client and a backup subsystem, the backup subsystem comprising a backup
server, a server backup program, a storage medium, and a copy of a
versioned sequence of a file stored in the storage medium, said versioned
sequence comprising a base version of a file in multiple compressed
segments and a predetermined number of delta files, the client having a
client backup program and a storage medium, the client and the backup
server are connected to each other by a communication link, comprising the
steps of:
establishing a backup session between the client and the backup server;
detecting, using the client backup program, a changed version of said file
at the client;
transmitting a changed version of said file in an uncompressed format,
using the communication link, from the client to the backup subsystem; and
differencing using the backup subsystem, the changed version of said file
and the multiple compressed segments of the base version of said file,
said differencing further including the steps of:
compressing the changed version of said file at the backup server one line
at a time;
comparing each compressed line of the changed version of said file with the
corresponding line of the base version of said file one line at a time;
determining whether a difference between the compressed line of the changed
version of said file and the compressed line of the base version of said
file has been detected;
decompressing the segment of the base version of said file, known as the
current segment, if a difference has been detected;
comparing the uncompressed current segment of the base version of said file
and the corresponding segment of the changed version of said file where
the difference has been detected;
creating a delta file which includes the differences detected;
determining whether the differencing procedure is back in synch at the end
of the current segment of the base version of said file; and
storing the changed version of said file in the compressed segmented
format.
2. A method for storing information in a client-server environment having a
client and a backup subsystem, the backup subsystem comprising a backup
server, a server backup program, a storage medium, and a copy of a
versioned sequence of a file stored in the storage medium, said versioned
sequence comprising a base version of a file in multiple compressed
segments and a predetermined number of delta files, the client and the
server are in communication with each other by a communication link,
comprising the steps of:
establishing a backup session between the client and the backup subsystem;
detecting a changed version of said file at the client;
differencing, using the client, the changed version of said file and the
multiple compressed segments of the base version of said file at the
client to create a delta file, said differencing further including the
steps of:
compressing the changed version of said file at the client one line at a
time;
comparing each compressed line of the changed version of said file with the
corresponding line of the base version of said file one line at a time;
determining whether a difference between the compressed line of the changed
version of said file and the compressed line of the base version of said
file has been detected;
decompressing the segment of the base version of said file, known as the
current segment, if a difference has been detected;
comparing the uncompressed current segment of the base version of said file
and the corresponding segment of the changed version of said file where
the difference has been detected;
creating a delta file which includes the differences detected;
determining whether the differencing procedure is back in synch at the end
of the current segment of the base version of said file; and
storing the changed version of said file in the compressed segmented
format;
transmitting said delta file, using the communication link, to the backup
subsystem: and
storing said delta file in the backup subsystem.
3. In a client-server environment having a client and a backup subsystem,
the backup subsystem comprising a backup server and a backup storage
medium, and where the client comprises a client storage medium, the client
and the backup server connected to each other by a communication link, a
method for reducing the storage requirements and transmission loads in
said client-server environment, comprising the steps of:
storing a versioned sequence of a file, using the backup storage medium, in
the backup subsystem, said versioned sequence comprising a base version of
a file in multiple compressed segments and a predetermined number of delta
files;
detecting a changed version of said file at the client;
differencing, using the client, the changed version of the said file and
the multiple compressed segments of the base version of said file to
create a delta file at the client, said differencing further including the
steps of:
compressing the changed version of said file at the client one line at a
time;
comparing, at the client, each compressed line of the changed version of
said file with the corresponding line of the base version of said file one
line at a time;
determining whether a difference between the compressed line of the changed
version of said file and the compressed line of the base version of said
file has been detected;
decompressing the segment of the base version of said file, known as the
current segment, if a difference has been detected;
comparing the uncompressed current segment of the base version of said file
and the corresponding segment of the changed version of said file where
the difference has been detected;
determining whether the differencing procedure is back in synch at the end
of the current segment of the base version of said file; and
storing the changed version of said file in multiple compressed segments;
transmitting, using the communication link, said delta file to the server;
and
storing said delta file in the backup subsystem.
4. In a client-server environment having a client and a backup subsystem,
the backup subsystem comprising a backup server and a storage medium, and
where the client comprises a client storage medium, the client and the
server connected to each other by a communication link, said client-server
environment comprising:
mean for storing a versioned sequence of a file in the backup subsystem,
said versioned sequence comprising a base version of a file in multiple
compressed segments and a predetermined number of delta files;
means for storing a copy of the base version of said file in the client;
means for detecting a changed version of said file at the client;
means for differencing the changed version of said file and the multiple
compressed segments of the base version of said file to create a delta
file at the client, said means for differencing further including:
means for compressing the changed version of said file at the client one
line at a time;
means for comparing, at the client, each compressed line of the changed
version of said file with the corresponding line of the base version of
said file one line at a time;
means for determining whether a difference between the compressed line of
the changed version of said file and the compressed line of the base
version of said file has been detected;
means for decompressing the segment of the base version of said file, known
as the current segment, if a difference has been detected;
means for comparing the uncompressed current segment of the base version of
said file and the corresponding segment of the changed version of said
file where the difference has been detected;
means for determining whether the differencing procedure is back in synch
at the end of the current segment of the base version of said file; and
means for storing the changed version of said file in multiple compressed
segments;
means for transmitting the delta file to the backup subsystem; and
means for storing said delta file in the backup subsystem so the base
version of said file can be updated at a predetermined time. |
|
 |
|
 |
Claims |
|
|
|
Description |
|
|
CROSS-REFERENCE TO RELATED APPLICATION
U.S. patent application Ser. No. 08/328,633, now pending, entitled SYSTEM
AND METHOD FOR REDUCING STORAGE REQUIREMENT IN.sub.-- BACKUP SUBSYSTEMS
UTILIZING DIFFERENCING, was filed on the same day, owned by a common
assignee and having the same inventor as the present invention.
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates in general to improvements in the field of computer
systems having backup/restore or archive/retrieve subsystems. More
particularly, this invention relates to a method and system for reducing
the storage requirements of backup subsystems in client-server
environments.
2. Description of the Background Art
In a data processing system, a backup/restore subsystem, usually referred
to as backup subsystem, is typically used as a means to save a recent copy
or version of a file, plus some number of earlier versions of the same
file, on some form of backup storage devices such as magnetic disk drives,
tapes, or optical storage devices. The backup subsystem is used as a means
of protecting against loss of data in a given data processing system. For
example, if an on-line version of a file is destroyed or corrupted because
of power failure, hardware, or software error, user error or some other
type of problem, the latest version of that file which is stored in a
backup subsystem can be restored and therefore the risk of loss of data is
minimized. Another important use of backup subsystems is that even if
failures do not occur, but files or data are deleted or changed (either
accidentally or intentionally), those files or data could be restored to
their earlier state thus minimizing the loss of data.
Therefore, it can readily be apparent that backup subsystems are and will
remain an important part of the field of data processing.
A closely related concept to the backup subsystem is a method and system
called archive/retrieve, usually referred to as an archive subsystem.
Archiving refers to making copies of files on lower cost storage such as
tape so that files can be deleted from more expensive technology such as
disk storage. Since disk storage is frequently being updated, an archival
copy also allows the state of a collection of data to be captured for
later reference, even if the primary copy of the data is not going to be
deleted. An example would be the archiving of a set of financial data at
the end of a fiscal period. Although the improved method of carrying out
the backup disclosed in this application is primarily described for a
backup system, it will be obvious to the person of ordinary skill in the
art of data processing that the systems and methods described herein are
also applicable to archive systems or other related storage management
systems.
At the present time, the majority of backup systems run on host systems
located in a data processing environment. Typically, a new version (also
referred to as changed version) of a file is backed-up based on a
predetermined schedule such as, at the end of each day, or after each time
that a file has been updated and saved.
Backup systems generally consume large amount of storage media because
multiple versions of large amounts of data are being backed up on a
regular basis. Therefore, those engaged in the field of data processing
and especially in the field of backup/restore systems are continuously
striving to find improved methods and systems to reduce the storage demand
in backup systems. Current backup systems typically utilize one or both of
the following methods to enable the storage of and retrieval of multiple
versions of a given file. These are: (1) the full backup method and (2)
the incremental backup method.
The full backup method is the most basic method used which requires the
backup of an entire collection of files, or a file system, regardless of
whether individual files in that collection have been updated or not.
Furthermore, in the full backup method multiple full versions of each file
are maintained on a storage device. Since maintaining multiple full copies
of many files consumes substantial amount of storage, some type of
compression technique is sometimes used to reduce the amount of data
stored. Compression techniques basically rely on the presence of
redundancy within the file, so called intra-file redundancy, in order to
achieve this reduction. The most common method is the use of a method of
file compression known as Lempel-Ziv method (also known as Adaptive
Dictionary Encoder or LZ coding) described in a book by T. C. Bell et.
al., titled Text Compression, pp 206-235. The essence of Lempel-Ziv coding
is that phrases are replaced with a pointer to where they have occurred
earlier in the text, thereby saving the storage space associated with
multiple occurrence of any given phrase. This is a general method which
can be applied to any file and typically results in compression ratios of
the order of between 2 and 3.
Incremental backup method is an alternative to full backup used in backup
systems where only those files, in any given collection of files, are
backed up which have been changed since the previous incremental or full
backup.
It is apparent to those skilled in the art that in any given backup system,
the higher the backup frequency, the more accurately the backup copy will
represent the present state of data within a file. Considering the large
volume of data maintained and continuously generated in a typical data
processing system, the amount of storage, time, and other resources
associated with backing up data are very substantial. Thus, those skilled
in the art are continuously engaged in searching for better alternatives
and more storage and time efficient systems and methods for backing up
data.
Aside from the compression technique which is heavily utilized to reduce
storage requirement in a backup system, there exists a quite different
method of achieving reduction in file size, known as delta versioning.
Delta versioning has never been used in any backup system.
Delta versioning which is also referred to as "differencing" or "deltaing"
relies on comparison between two files where multiple version of a file is
saved in a form of a "base" file, also called a "base version" of a file,
together with predetermined number of small files which represent only the
changes to the base file. The small files, also referred to as "delta"
files or "difference" files, contain the difference or delta from the base
file. Delta files are generated as a result of comparing the base file
with a later (newly arrived) or an earlier version of the base file. Thus
this method of storage reduction exploits redundancy between files, or
"inter-file" redundancy, in order to achieve reduction in storage
requirement. This method which is used in the software art of Source Code
Control Systems, discussed in a reference below, can provide substantial
storage saving in backup systems, since frequently the selection of a file
for incremental backup occurs after a small change has been made to that
file. Therefore, since many copies are frequently made in backup systems
to files that differ only slightly from one another, the differencing
method offers great potential for substantial reduction in the amount of
data stored in backup subsystems.
At the present time none of the backup systems that use compression
techniques utilize delta versioning. Moreover, no one has ever invented a
method and system allowing the use of compression and delta versioning
together in the same backup system.
Delta versioning falls into two general classes: one is where the base file
is the oldest version of a file and the delta files represent newer
versions. This method is referred to as "forward" deltas. The other is
where the base file is the latest version of a file and the delta files
represent older versions. This method is referred to as "reverse" deltas.
The "reverse" delta is the more common method because usually the most
utilized version of a file is the last version created.
A technical paper by M. J. Rochkind, titled "The Source Code Control
System", IEEE Transaction on Software Engineering, Vol. SE-1, No. 4,
December 1975, PP 364-370, teaches a software tool, known as source code
control system (SCCS) which is designed to help managing changes to a
source code (source program) in the field of software development tools.
In SCCS environment, every time a module (file) is changed the change is
stored as a discrete delta where the space required to store a delta is
only slightly greater than the amount of text inserted by that delta.
However, Rochkind does not teach or suggest the use of delta files in a
backup and archiving subsystem in either a central or a client-server
environment as a means for reducing the storage requirements of such
subsystems.
U.S. Pat. No. 4,912,637 issued on Mar. 27, 1990 to C. R. Sheedy et al.,
teaches a system for preserving, generating, and merging various versions
of the same file by a modified delta method. Sheedy teaches using an
indexed line file where every line active in any version of a given file
is stored, together with a variant history file where the history of the
status of each line in various versions is recorded. Using these two
files, any desired version of a program may be generated directly without
the need for creating any of the intermediate versions. However, Sheedy
does not teach or suggest the use of this modified method in backup and
archiving systems in either a central processing or a client/server
environment as a means for reducing the storage requirement of a backup
system.
U.S. Pat. No. 5,263,154, issued on Nov. 16, 1993 to L. E. Eastridge et al.,
teaches a method and system for incremental backup copying of a file in a
data processing system which minimizes the suspension of the data
processing system during such backup copying. This is done by first
physically backing up a data set on a storage subsystem on a scheduled or
opportunistic basis. Thereafter, creating side-files of the data set
modified. The side-files are then used in the next scheduled or
opportunity to update the backed-up data set. However, Eastridge does not
teach or suggest the use of delta files as a means of minimizing storage
requirement in a backup and archiving subsystem in either a central
processing environment or a client-server environment.
U.S. Pat. No. 5,278,979 issued on Jan. 11, 1994 to R. D. Foster, et al.,
teaches a method and system in the field of software code development
known as Single Entity Versioning where by creating and maintaining a
unique version identification and a control data file, multiple versions
of source data is efficiently stored in a single entity. However, Foster
does not teach or suggest the use of delta versioning or Single Entity
Versioning as a means of minimizing storage requirement in a backup and
archiving subsystem in either a central processing environment or a
client-server environment.
Considering that the amount of the data generated on the daily basis by the
computers is growing at a very fast rate, there is a need for an improved
and innovative method and system to reduce the storage requirements of
backup systems in central data processing systems and further in
client-server environments which as will be discussed below present unique
backup issues.
Backup Subsystems in a Client-Server Environment
Recently, the emergence of low cost local area networking, personal
computer, and workstation technology has promoted a new type of data
processing architecture known as the "client-server" system or
environment. A client-server system 10, as shown in FIG. 1, typically
consists of: (1) client computers (also referred to as clients) 11 such as
personal computers or workstations with their own local storage medium 12
such as disk storage devices; (2) a local area network (also referred to
as LAN or network) 13 such as an Ethernet or a Token Ring which links the
clients to the LAN server(s); and (3) one or more LAN server computers 14
such as a personal computer or perhaps a workstation with its own local
storage mediums 15 such as disk storage devices, tape storage and/or
optical storage devices.
In a client-server environment, the majority of the data processing is
usually carried out at the clients which are connected by a local area or
other network to a LAN server. The LAN server usually contains various
programs or data which are commonly used by many of the clients. Computer
users which usually use clients to carry on their data processing tasks,
are generally in control of the client computers whereas the LAN server(s)
is usually administered by an expert administrator of a data processing
(computing) center.
The client-server environment presents a number of major issues as relates
to data processing, integrity, and backup of such data. One major concern
in the client-server environment is that a substantial amount of important
data is located on client subsystems which lack the security, reliability
or care of administration that is typically applied to the server
machine(s). There is a further concern that data may accidentally be lost
from a client computers, because the users of such computers do not take
time and necessary care to back up the data on a regular basis. There is
yet another concern that the amount of data residing on the clients are so
substantial that even if a client-server backup subsystem could be
developed to attempt to backup all these data, the amount of backup
storage required to save all the data on the clients would be inordinate
and impractical. The lack of an efficient backup system and method has
been a major barrier to the adoption and rapid growth of client-server
technology despite its many attractive features.
Recently a number of client-server backup systems have been developed to
alleviate some of the concerns listed above. An example is an IBM's ADSM
(ADSTAR Distributed Storage Manager) product. This technology overcomes
some of the deficiencies mentioned above by making backup copies of the
client data on a backup server. The client copies are made automatically
without user involvement and are stored on storage devices which are
administered by the backup server.
A typical client-server backup subsystem such as ADSM operates as follows.
In the client computer a program exists, known as the client backup
program, which at pre-specified or periodic times is activated and makes
contact with a program residing on the backup server, known as the server
backup program. After establishing contact and establishing
authentication, the server backup program then consults "policy data"
which instructs the server backup program as to what sort of a backup
operation should occur and which files on the client computer are the
subjects of the current backup. It then searches all or a subset of files
on the client computer, determining which files should be backed up. For
example, a data file which has changed since the backup program was last
run may cause that file to be selected for the backup operation. After
selecting the files to be backed up, the client backup program transmits
those files, using the LAN, to the server backup program. The server
backup program then makes an entry in a "backup catalog" for each file
received and then stores those files on storage devices attached to the
backup server.
The server backup program also carries out several other important
operations. One such operation is the maintenance of its storage pools.
For example, backup copies of files that were made many months ago may be
moved from disk storage to tape storage in order to reduce storage costs.
Another important function of the client and server backup programs occurs
when the user requests the restoration of a file. The client backup
program contacts the server backup program which consults its backup
catalog to establish the location of the backup copy of the file. It then
returns that file across the network to the client computer which in turn
makes it available to the user.
Hardware which is typically needed for implementing a backup system in a
client-server system includes: one or more server computers such as PC or
workstations and storage mediums such as IBM 3390 magnetic storage system,
IBM 3494 tape storage library or IBM 3595 optical library. These libraries
which provide automated mechanical mounting and demounting of tape or
optical cartridges into read/write drives and retrieve them from or
replace them within the storage shelves are sometimes referred to as
"jukeboxes".
Despite the recent improvements made in the field of client-server backup
systems, several shortcomings have remained in all client-server backup
systems including ADSM. One of the shortcomings, as mentioned earlier, is
that the very large number of files on the clients now being regularly
backed up tend to generate very large amounts of data resulting in large
storage requirements and therefore substantially more cost in backing up
data. Although systems such as ADSM compress this data on the storage
devices, the amount of data remains very large. A second difficulty that
is being observed is that the local area network technology is frequently
unable to complete transmission of all of the changed files, even in only
an incremental backup, to the backup server during the designated period
for backup operations (e.g., a night shift). This is due to the bandwidth
limitation of the communication network (which might include low speed
remote telephony data links) and large amount of data that has to be
transmitted from numerous clients to the backup server.
It is apparent now that implementation of an efficient backup subsystem in
a computer processing environment is a formidable task and in a
client-server environment poses significant challenges of its own.
Therefore, there is a need for a new and novel backup method and system in
a client-server environment that not only substantially reduces the
storage requirement of backup subsystem but also minimizes the burden on
the communication link between the clients and the backup server. The
present invention addresses these two major deficiencies currently present
in all client-server backup subsystem by providing alternative methods and
systems which can be used to reduce the amount of data storage required in
a client-server backup subsystem and reduces the burden on the bandwidth
of the transmission network.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide improved
backup and archiving methods and subsystems in a data processing
environment.
It is a further object of the present invention to provide improved backup
and archiving methods and subsystems in a client-server environment.
It is a further object of the present invention to reduce network
transmission cost in a client-server environment.
It is another object of the present invention to reduce data processing
loads on a backup server in a client-server environment.
It is another object of the present invention to reduce the transmission
bandwidth requirement in a client-server environment.
It is another object of this invention to provide a method and system for
combining compression and differencing so that both intra-file and
inter-file compression can be exploited together in a backup subsystem.
It is yet a further object of the present invention to provide a method and
system for combining the difference method with compression methods in a
client-server subsystem, thereby reducing the network transmission cost.
It is another object of the present invention to provide a method for
combining the difference method with compression methods in a
client-server subsystem, thereby reducing the storage requirement in the
backup subsystem of a client-server system.
It is another object of the present invention to provide a method for
combining the difference method with segmented compression methods in a
client-server subsystem, thereby reducing the storage requirement in the
backup subsystem of a client-server system.
The foregoing objects are achieved by the invention disclosed herein.
Briefly stated, in one embodiment of the invention, a file, called the
"new" file (also referred to as new version of the base file or changed
version of the base file), is recognized to have been changed at the
client, and is then transmitted to the backup server (referred to as
server). At the server the new file is differenced against the previous
latest version of the file, called the "previous" file (also referred to
as "base" file) which is in the compressed form to produce a file referred
to as new delta. That is, the differencing is carried out on the
compressed version of the base file and the changed version of the base
file without decompressing the entire base file. This new delta can simply
be stored along with the "new" file or the "base" file depending on
whether reverse or forward delta versioning is used, respectively. Based
on controlling policy management, if a fixed limit on the number of delta
files is enforced, then the oldest delta is deleted.
In another embodiment of the present invention, in a client-server
environment, the differencing is carried out on the compressed version of
the base file and the changed version of the base file without
decompressing the entire base file by the client rather than the server.
However, in order for the client to carry out the differencing operation,
it needs to keep a copy of the base file at the client. When the base file
is modified on the client, then a new delta file is created by the client
and transmitted to the server. The server may simply save the new delta
and then use it at a later time to modify the compressed base file in the
backup subsystem, or may apply it immediately against the base file to
create a copy of the new file.
Alternatively, instead of storing a copy of the compressed base file at the
client for the purpose of differencing, the compressed base file may
simply be transmitted to a client whenever the client needs to modify the
base file. Once the compressed base file is modified on the client, a new
delta file is created and transmitted back to the server to be used and
stored by backup subsystem. Note that by carrying out the differencing at
the client rather than the server, the size of the file that has to be
sent back to the server is substantially reduced, thereby substantially
reducing the burden on the transmission network.
By using differencing and segmented compression substantial reduction in
storage requirement, network transmission bandwidth, and CPU efforts
expended on compression/decompression of the base file can be achieved.
The use of differencing method and compression can be carried out together
either at clients or server or at both the clients and the server.
In the past, the use of compression and differencing together on the same
file was considered not possible since small change to a file could result
in large changes to the compressed version of the file. This is because a
change of one byte in a file can cause arbitrary changes in the remainder
of the compressed file. Thus, if two files differ in the middle by only
one character, the second half of compressed version of each file could
turn out to be totally dissimilar. Furthermore, such a difference could
result in an arbitrary change in the length of compressed file. Because of
this problem, compression and differencing has never been used together in
any backup or storage subsystem. However, this problem, according to the
invention presented here, is solved by compressing the base file into
multiple segments.
According to this embodiment of the present invention, the base file is
compressed into multiple segments. A new file which is detected is also
compressed, and as is being compressed, the base file and the newly
compressed file are then compared without decompressing. As soon as a
difference between the two files is detected, the appropriate segment of
the base file is decompressed and compared with the corresponding
uncompressed portion of the new file so a delta file can be generated.
When the comparison between the appropriate segments are completed, if the
differencing method is "back in synch" (i.e., in the state of not
detecting any more differences), then the comparison of the compressed
version of the base file and the new file is continued. If the
differencing method is not back in synch at the end of the segments under
comparison, comparison of the uncompressed segments of data in the base
file and the new file is continued possibly to the end of the files.
The innovative embodiment stated in the previous paragraph is applicable in
computing the deltas when differencing is carried out either at the server
or client or both. Furthermore, this embodiment teaches a novel method and
system to use both segmented compression and differencing together in a
backup and archiving system. This is a significant breakthrough in the art
of backup systems since current backup systems such as ADSM only make use
of file compression but have not been able to implement a method that uses
compression and differencing together.
It should be noted that whereas compression typically results in a space
saving factor of two or three, the space saving when utilizing
differencing can be much larger. For example, if n versions of a file are
saved and they contain only small differences (e.g., a few lines are
changed or appended in each file), then the space saving factor can
approach n.
BRIEF DESCRIPTION OF THE DRAWINGS
For a fuller understanding of the nature and advantages of the present
invention, as well as the preferred mode of use, reference should be made
to the following detailed description read in conjunction with the
accompanying drawings.
FIG. 1 is a schematic diagram of a typical client-server environment;
FIG. 2 is a schematic diagram of a client-server environment having a
backup subsystem;
FIG. 3A is a flow chart illustrating the backup operation at the client of
the preferred embodiment of the present invention;
FIG. 3B is a flow chart illustrating the backup operation at the server of
the preferred embodiment of the present invention;
FIG. 4 is a flow chart illustrating implementation of diff(,,) operation;
BEST MODE FOR CARRYING OUT THE INVENTION
The following description is the best mode presently contemplated for
carrying out the invention. This description and the number of alternative
embodiments shown are made for the purpose of illustrating the general
principle of the present invention and is not meant to limit the inventive
concepts claimed herein.
With reference now to FIG. 2, there is shown a client-server system 20
having a backup subsystem. System 20 typically includes a plurality of
client computers 21 with their own local storage medium 22 such as disk
storage devices. The client computers (clients) 21 may typically be
personal computers of the type having a system unit (not shown) which
includes CPU (processor), I/O control, and semiconductor and magnetic
memories and DOS, OS/2, or Apple Macintosh operating systems. The client
computers 21 may further be workstations of the type having AIX, UNIX or
equivalent operating systems. These operating systems are well known to
those skilled in the art of data processing and need no further
description.
Still referring to FIG. 2, the client-server system 20 further includes a
local area network (LAN) 23 such as Ethernet or a Token Ring which
provides the communication link between the clients 21 to the backup
server(s) 25.
Backup server computer 25 may typically be a personal computer of the type
having a system unit (not shown) which includes CPU (processor), I/O
control, and semiconductor and magnetic memories and DOS, OS/2 or Apple
Macintosh operating system. It may also be a workstation having a system
unit and UNIX or AIX or equivalent operating system. It may also be a
large system running the AS/400, VM or MVS operating systems. Computer 25
further has its own local storage mediums 26 such as disk storage devices
27, optical library (storage) devices 28, or tape library (storage)
devices 29. In a client-server system 20 shown in FIG. 2, backup system
usually resides at the backup server 25. A typical backup system that
resides at the server is IBM Advanced Distributed Storage Manager (ADSM)
which has been explained in detail in the background section of this
application. The operation and physical implementation of personal
computers, workstations, disk storage devices, optical library, tape
library and their constituents are well known to those skilled in the art
of data processing and requires no further description.
General Notation
We now state a general notation for describing "base file" and "delta file"
storage which will be applicable to all the embodiments described herein.
We will assume that a versioned sequence of files which are stored can be
represented in the general form:
[d.sub.-- 1, d.sub.-- 2, . . . , d.sub.13 n, F, d | | |
