HP 3000 Manuals HP 3000 Manuals
DUMP/DAT Enhancements by Jim Miller and Ray Terry Commercial Systems Division The DUMP facility and the Dump Analysis Tool (DAT) have been enhanced for this MPE/iX release. DUMP and DAT now use a very efficient data compression algorithm to reduce the amount of disk and tape space required to hold a system dump. On the average, 50 percent less dump space is used than previously. On high-end systems, the time required to take a dump to tape may be reduced by up to 25 percent of the previous version of DUMP. Since XL Release 1.1, DUMP and DAT have used a run length encoding (RLE) data compression algorithm to reduce the size of dumps. The RLE algorithm simply looks for sequences of consecutive identical characters and replaces them with the character and the length of the run. Beginning with this MPE/iX release, DUMP and DAT use a variation of the LZ data compression algorithm. LZ stands for the originators of this compression technique, Abraham Lempel and Jacob Ziv. A 1967 paper describing a semi-adaptive dictionary coding technique closed with the remark that better compression could be obtained by "replacing a repeated string by a reference to an earlier occurrence." In 1977, Jacob Ziv and Abraham Lempel described an adaptive dictionary encoder in which they "employ the concept of encoding future segments of the input via maximum-length copying from a buffer containing the recent past output." Almost all practical adaptive dictionary encoders are encompassed by a family of algorithms derived from Ziv and Lempel's work. This family is called Ziv-Lempel coding, abbreviated and transposed as LZ coding. Use of the LZ coding, or data compression, in DUMP and DAT result in smaller dumps. LZ dumps are approximately 50 percent smaller than a comparable RLE dump. DAT has also been enhanced to be able to read LZ compressed dumps. DAT is now able to read either the older RLE dump files or the new LZ dump files. The DUMPINFO ALL command within DAT displays whether the dump is compressed with RLE compression or with LZ compression. If an RLE compressed dump tape is encountered while doing a GETDUMP in DAT (reading a dump tape to disk), DAT converts the dump into LZ compressed format. This allows you to still obtain the disk space savings when working with old-style RLE dump tapes.