Текст 10000 знаков Архивация данных. Архиватор ARJ / реферат архиватор без перевода
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Тульский государственный университет
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“Архивация данных. Архиватор ARJ”
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Тула 2003
The head-line of the text is ”Archiver ARJ”. The author of this article is unknown. This article is a part of the help file of the archiver ARJ. The purpose of the article is to learn the user to work with the archiver. The author gives the reader some information on use of the archiver ARJ.
The author starts by telling the readers about terminology used in article. The author acquaints the reader with such terms as arhive, ARJ file, compression, extraction, uncompression etc.
ARCHIVE - This is a file containing one or more files in a compressed or noncompressed state and containing file related information such as filename and date-time last modified, etc.
ARJ FILE - This is an archive created by ARJ.
BACKUP TYPE ARCHIVE - This is an ARJ archive that has the internal backup flag turned on.This feature is obsolete after ARJ version 2.50a.Current and future versions of ARJ will ignore the special flag set on file backups.Replacing this backup archive scheme is the chapter archive scheme.
CHAPTER ARCHIVE - This is an ARJ archive that contains one or more backup chapters. A chapter archive stores a full backup and a series of incremental backups in a manner that saves disk space by not storing duplicate files.A chapter archive allows the user to recover a directory of files as they were when the backup chapter was made.In other words, if daily backup chapters of a source directory were made from Monday to Friday, one could recover the source directory as it was on Wednesday.
COMPRESSION - The process of encoding redundant information into data requiring less torage space.
COMPRESSION PERCENTAGE/RATIO - The percentage compression reported by ARJ is a variation of one of the TWO standard methods of expressing compression ratio in the technical literature.ARJ uses the compressed size / original size ratio.The other method is the inverse ratio. When ARJ reports 96% as the compression ratio, which means that the compressed file is 96 percent of the original size (very little compression).Other archivers use their own methods. LHARC uses the same ratio as ARJ.
EXTRACTION or UNCOMPRESSION - The processing of recreating the exact information that was previously compressed.
SELF-EXTRACTION MODULE (SFX) - This is an archive that is an executable file that is capable of extracting self-contained files.
TEXT MODE - In text mode, ARJ inputs the file using the C library text mode which translates the carriage return, linefeed control characters of MS-DOS to a single linefeed character. This saves space and provides the option for cross platform file extraction. On another platform, the host C library would change the single linefeed to the host text newline separator sequence.In addition, for platforms such as PRIMOS which set bit 8 in ASCII text characters, ARJ sets/resets bit 8 according to the platform extracted to. When extracting a text mode file to the same type of platform archived from, ARJ will NOT strip the 8-bit text to 7-bit text.
VOLUMES - These are ARJ archives that are in sequence and have been reated by a single ARJ command.Files in the volumes may span volumes in a split format.These volumes are usable archives.
The text divided into 8 parts. The first part is devoted to a terminology used in the text.
The second part is about features of ARJ. This is such features as the ability to process and archive up to 65,000 files at one time, the ability to select files for archival by type and attribut, support for Windows 95 long filenames in the Windows 95 GUI DOS environment, 32 bit CRC file integrity check, DOS volume label support, empty directory support, test new archive before overwriting the original archive option, string searching with context display within archive files, built-in facility to recover files from broken archives, multiple volume self-extracting archives, password option to encrypt archived files etc.
The third part is devoted to a arciver benchmarks. This is information for those who plan to publish benchmark test results comparing ARJ with other file archivers.
The ARJ -jm compression is intended to demonstrate the best that ARJ can do in terms of size reduction. However, the ARJ -jm1 compression is almost as good in terms of size reduction. The ARJ -m2 compression is intended to compete with LHA 2.12.The ARJ -m3 compression is intended to compete with PKZIP 1.10. The ARJ -e option is necessary during size benchmarks because ARJ by default stores the entire specified pathname in the archive as opposed to other archivers which strip path specs. Example command: ARJ a benchmrk -e -jm1 files\*.
The very size of the ARJ runfile adds significantly to the compression and
extraction times when testing smaller archives.
The fourth part of the text devoted to the technical notes.A detailed technical description of the ARJ archive format is available in the UNARJ distribution archive. This archive contains a description of the archive header formats as well as C source code for an ARJ archive extractor and lister program. This source code has been made portable to several platforms including UNIX, NEXT, DOS, and AMIGA.
Usually programs process information included in tables. In most cases these tables present not formlees mass of numerical values, but data elements with important structure ties.
In the primary case table may be presented as a simply linear list of elements. All information about its structural property can be received from the answers onto following questions: What element is situated in the first place of the list? What element is last? What elements are placed before given element and what elements are situated after it? How many elements are there in the list? If you will give answers on these questions, you will get wide knowledge about data structure.
The fifth part tells about the system requirements. The ARJ software package is designed to work under DOS versions and DOS boxes from 2.11 and above. However, certain features have an OS limitation under DOS versions prior to DOS 3.0: access to the self-extractors and to the built-in help information. To get the ARJ help screens and the ARJ self-extraction creation to work,ARJ.EXE MUST be located in the CURRENT directory. This limitation exist only in newer versions of ARJ to save on memory usage.
The sixth and seventh parts formulate about the important notes and chapter archiver. When using the "-w" working directory switch, ARJ does not check on space availability before overwriting the original archive if it exists. Be sure that you have enough disk space for the new archive before using the "-w" switch. If ARJ aborts in this situation because of disk space, ARJ will keep the temporary archive.
By default, ARJ does not see hidden or system files. ARJ will process system and hidden files when you either specify the "-a" switch or use the new "-hb" switch.
Like LHARC and PKZIP, ARJ requires extra disk space to UPDATE an archive file.ARJ will backup the original archive while it creates the new archive, so enough room must be available for both archives at the same time.
Unlike PKZIP, ARJ does not require additional work space when CREATING a new archive.
Currently, ARJ will not extract overwriting a readonly file unless the "-ha" option is specified.
This feature is useful when backing up frequently modified files. This new feature is designed to make the backup management of a directory or application easier. It allows the taking of a sequence of backup "snapshots" of a changing application or directory. A chapter archive can provide major space savings over using many separate archives for backup.
For example, a chapter archive could be used to keep track of the state of a Windows 3.1 directory. An initial chapter backup could be made of the original Windows 3.1 installation. Then, whenever a new software package is installed, a new chapter snapshot could be taken.
ARJ ac winback c:\windows\*.* -r -a1 -jt1
If a Windows problem develops, one can restore Windows back to any particular chapter for testing.
ARJ x winback c:\ -y -jb5 restore chapter 5
This chapter feature is especially useful for software developers. Since a chapter archive can contain up to 250 chapters, the change history of a software source directory for a period of weeks can be maintained in a single archive. You could take four chapter snapshots per day for over two months to a single archive. This
would allow you to restore your source directory to one or more points in time to help you track the appearance of software problems.
See the "-jb" option for more details.
The limitation on the maximum number of chapters is an internal resource issue as well as a "recommendation" to build a new archive. Making thousands of modifications
to important backup data is a somewhat risky activity.
In conclusion the article reads about ARJ errors situatuons. ADD:
If a user specified file is not found during an add, ARJ will continue processing, and will keep the archive and terminate with an error condition. Note that files specified within an ARJ listfile that are not found during an add will NOT trigger an error unless the "-hl" option is also specified.
In a disk full condition or any other file i/o error, ARJ will promptly terminate with an error condition and delete the temporary archive file unless the user has specified the "-jk" switch.
MOVE:
ARJ will only delete files that have been successfully added to the archive.If you have specified the "-jt" (test) switch, ARJ will abort on any error.If you specify the "-jk" switch, ARJ will keep the temporary archive upon an abort.
EXTRACT:
In a disk full condition or any other file i/o error, ARJ will promptly terminate with an error condition and delete the current output file.
CRC ERRORS OR BAD FILE DATA:
In the case where an ARJ archive has been corrupted, ARJ will report a CRC error or a Bad file data error. These corruptions can be the result of an unreliable diskette, a computer memory problem,a file transfer glitch, or incompatible CACHING software. Most of these errors are the result of file transfer glitches and bad diskettes. A few are the result of an incompatible interaction with SUPER PCKWIK 3.3 advanced diskette support or Windows 3.x.
Individual files in an archive that are affected by the CRC or Bad file data errors CANNOT be recovered. Other files in a damaged archive can be recovered by extracting them using the "-jr" option.
However, with some preventative action as in the use of the –hk option to generate data damage protection file, slightly corrupted ARJ archives can be fully repaired and all damaged files recovered.
CRITICAL ERROR HANDLER:
ARJ sets up an interactive critical error handler to handle DOS critical errors like "sector not found" and "drive not ready". When a critical error occurs, ARJ will prompt the user with the message "Retry Y/N?". The user may retry the failed operation by pressing "Y". Pressing "N" will fail the operation or abort to DOS depending upon the version of DOS. The user can press Control BREAK to abort to DOS.