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Wrap

Text File | 1994-06-18 | 12.8 KB | 250 lines

__________________________________________________________________________ CRUSH v1.0 Readme File __________________________________________________________________________ CRUSH is a new shareware product from the same author as the awarding winning DOS file manager PocketD Plus ("BEST UTILITY 1992" PsL and joint runner-up WHAT PC's "BEST UTILITY 1994" with Stacker v4.0 & Dr Solomon's Virus Toolkit). PocketD Plus has been upgraded to v4.1 to support CRUSH. >>>>>>>> WHAT IS CRUSH ? CRUSH is a compression tool that can work with your existing archiver to dramatically reduce the size of archives created. In many instances CRUSH achieves an average 2:1 advantage over PKZIP and 8:1 over uncompressed files. However CRUSH is neither slow nor inconvenient to use. The command-line syntax is essentially similar to PKZIP, but with many powerful extensions. CRUSH will work with any of PKZIP, ARJ, LHA, ZOO and HA. The archivers PKZIP and ARJ are Shareware and require registration payments. LHA, ZOO and HA are Copyrighted Freeware. >>>>>>>> HOW DOES IT WORK ? The principal behind CRUSH is not new, but is not exploited by common archivers. It takes advantage of the fact that big files will generally compress better than small files, a property used by Unix users who follow the maxim "always tar before compress" (joining files together before compressing). CRUSH will automatically do this for you, generating a single file with an extension "CRU" that it compresses using your chosen archiver (default PKZIP). Extraction of files is then very easy: UNCRUSH works in the same way as PKUNZIP. CRUSH gets the most from this compression trick by taking it one step further and intelligently ordering the files before joining them. It does this by recognising file types, reading them where needed (to recognise 7 different compressed header types), and orders them accordingly. This yields a very good default fast compression. Given more time CRUSH can do better by optionally performing a series of trials to squeeze out extra compression. The algorithm for this uses an intelligent re-ordering mechanism which analyses the results of each intermediate test in order to determine which ordering to try next. This was developed over a period of 2 years where it was successfully used to reduce the file size of PocketD Plus to the absolute minimum. This technique allows CRUSH to find near optimum compression in a few minutes that might take hundreds of years using more brute-force methods! >>>>>>>> HOW GOOD IS IT? Using CRUSH would not be worthwhile if it yielded a narrow advantage over existing archivers, but in many situations it is capable of delivering dramatically better compression. The results below are genuine random tests performed on files taken from a 250-user PC server and from a publically available CD-ROM. All archivers were run with maximum compression, except CRUSH which was set to minimum. The figures for Stacker and DoubleSpace are commercially quoted performance figures for comparison, not test results. All the figures quote the compression ratio in terms of disk space used rather than actual file sizes (8k cluster assumed) to allow comparison with Stacker 4.0. This does not significantly affect the comparison between the other archivers (Figures for ZOO are not quoted as it uses the same compression algorithm as LHA). >>>>>>>> (1) CRUSH at its BEST -- Working with small data files CRUSH works best with collections of similar small files. In a block test of 128 directories (each compressed to its own archive) containing 2349 wordprocessing files (total 33 Megabytes), CRUSH with minimum optimisation generated the following average compression ratio, calculated by: Ratio = (disk space used by uncompressed files)/(space after compression) Ratio CRUSH+HA ±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±± 7.95 CRUSH+PKZIP ±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±± 7.82 CRUSH+ARJ ±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±± 6.16 CRUSH+LHA ±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±± 5.46 HA 0.98 ±±±±±±±±±±±±±±±±±±±±±±±±±±±±±± 4.32 PKZIP 2.04g ±±±±±±±±±±±±±±±±±±±±±±±±±±±±± 4.11 ARJ 2.3 ±±±±±±±±±±±±±±±±±±±±±±±±±±±±± 4.18 LHA 2.11 ±±±±±±±±±±±±±±±±±±±±±±±±±±±±± 4.12 Stacker 4.0 ±±±±±±±±±±±±±±±±±± 2.50 DoubleSpace ±±±±±±±±±±±±± 1.90 Orig Files ±±±±±±± 1.00 CRUSH clearly yields a substantial improvement in all cases. Its worst result is with LHA, but this is still much better than any non-CRUSH compression. >>>>>>>> HOW FAST IS CRUSH ? These results might be of limited value if the cost in time for compression was great. The chart below shows the compression times for 6 wordprocessing files, totalling 150k on a 486SX25 with a ramdrive: Seconds CRUSH+HA ±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±± 11.04 CRUSH+ZIP ±±±±±±±±±±±±±±±±±± 3.51 CRUSH+ARJ ±±±±±±±±±±±±±±±±± 3.30 CRUSH+LHA ±±±±±±±±±±±±±±±±±±± 3.68 CRUSH+ZOO ±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±± 8.57 HA 0.98 ±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±± 10.98 PKZIP 2.04±±±±±±±±±±±± 2.26 ARJ 2.3 ±±±±±±±±±±±±±±±± 3.02 LHA 2.11 ±±±±±±±±±±±±±±±±± 3.30 ZOO 2.1 ±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±± 7.85 From these compression and speed results it can be seem that CRUSH with HA 0.98 offers the best overall compression, but that CRUSH with PKZIP or ARJ does nearly as well, but are both over 3 times faster. >>>>>>>> (2) CRUSH at its WORST -- Working with large or dissimilar files CRUSH has the most difficulty when given large or dissimilar files. A good example of this might be a Shareware release file which contains a mixture of documents, executable and other files. A test with minimum optimisation on 40 random archive files from the ASP-CD ROM (17 Megabytes) yielded the following results: Ratio CRUSH+HA ±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±± 2.84 CRUSH+ZIP ±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±± 2.80 HA 0.98 ±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±± 2.76 PKZIP 2.04g ±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±± 2.70 ARJ 2.3 ±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±± 2.70 LHA 2.11 ±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±± 2.66 Stacker 4.0 ±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±± (2.50) Doublespace ±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±± (1.90) Orig Files ±±±±±±±±±±±±±±±±±±±± 1.00 Here, improved performance is somewhat less than the previous example, corresponding to a 3.7% improvement with PKZIP. Many of the compressions yielded a 0% improvement because although the newly compressed file may have been smaller, it still occupied the same number of disk allocation units (clusters). The average improvement in actual file size (important for file downloads) was 5.6% for PKZIP, with a low of 1% and high of 28%. The figures for Stacker and DoubleSpace are bracketed as they almost certainly greatly overestimate their performances in these cases. >>>>>>>> I USE STACKER. IS THERE MERIT IN USING CRUSH ? On the fly compression provided by products such as Stacker offers convenient but low performance compression. Stacker may make your 100 Mb drive look like 250 Mb, but CRUSH may make it look like 800 Mb. On the fly compression also has performance penalties and there is a question mark over its safety. Of course, CRUSH can safely be used in conjunction with Stacker to obtain the highest possible net space saving. >>>>>>>> IS IT WORTH MY WHILE USING CRUSH ? If your PC is loaded with applications, but little data, then CRUSH may not help that much. If your PC or PC server typically holds many source files, database files or wordprocessing documents, then the advantages might be enormous. A beta test site for CRUSH had vast numbers of wordprocessing files for which CRUSH was able to achieve a compression ratio of 16:1 where PKZIP had only yielded 6:1. >>>>>>>> ARE THERE ANY DRAWBACKS IN USING CRUSH ? CRUSH requires a significant quantity of temporary filespace whilst compressing. In the default mode it will typically need 3.0 Mb of temporary disk space available in order to compress 2.0 Mb of files. The same is true when uncompressing. To extract a single 1k file from an archive that contains 1 Mb will require 1 Mb of temporary file space. CRUSH will not allow the user to update a single file or group of files within a CRUSH archive. The entire archive must be re-created. It will therefore be less useful to users who want to regularly update the same archive to reflect frequent changes. CRUSH is new and therefore many 3rd party programs will not yet be able to search or view CRUSHed files. However these will appear, and the very capable PocketD Plus v4.1 can already do this, allowing viewing of files inside CRUSH archives with the same ease as any other archive type. >>>>>>>> ARE THERE ANY OTHER ADVANTAGES IN USING CRUSH ? CRUSH supports features such as recursive directory searching, storing of paths, archive comments, file inclusion/exclusion by name and date, as other archivers. However CRUSH also adds its own special facilities. 1. On-line Selection of Files CRUSH allows the user to select files for archiving from an on-line prompt, rather than forcing files to be always explicitly specified in the command-line. This allows the user to issue commands such as "Search the drive for files matching *.C modified during the last 6 days and present each matching name for acceptance or rejection". 2. Automatic Archive Name Generation CRUSH has an option to automatically generate unique archive names based on the current date and time. This is an ideal mechanism for creating multiple generation backups. 3. Relative and Backup Date Testing Unlike programs such as ARJ and PKZIP, CRUSH allows relative dates to be specified, e.g. "CRUSH -r:-4 BACKUP" will search for and compress files modified today or during the previous 4 days. CRUSH also allows a "compress-since-last-backup" feature by allowing the comparison with a named file date and time, e.g. "CRUSH -:.LASTBAC BACKUP" will compress files modified since the date and time of the file LASTBAC. 4. Safer Decompression UNCRUSH will helpfully tell you that a file to be de-compressed already exists, and also if it is older, newer or identical. 5. CRUSH Supports Proper Wildcards Unlike DOS (and PKZIP etc.), CRUSH supports proper wildcards. e.g. the DOS wildcard *FRED.* would match a file called JOE! CRUSH correctly implements "*" as any sequence of characters (including none) and "?" as a single character. 6. User-specified Thresholds CRUSH can be set to create a conventional archive where it cannot improve by a specified percentage improved compression. >>>>>>>> HOW DO I COMPARE CRUSH AND PKZIP (OR OTHER) PERFORMANCE ? CRUSH has a convenient option "-c" which will run the chosen archiver as well as performing a CRUSH archive operation. The results of this are displayed as a bar graph (or table, if required). >>>>>>>> HOW MUCH EXTRA WILL MAXIMUM CRUSH COMPRESSION GIVE ? The cost in time for turning on extra optimisation in CRUSH is large. Adding the -f option will probably increase compression time by a factor of 5-10 times the default minimum, probably yielding between 1%-10% extra compression. However if you want to compress to the minimum, then you may be happy to leave it overnight with -f200!