HP 3000 Manuals HP 3000 Manuals
Database Files Database elements are stored in privileged MPE/iX disk files. In addition to the root file which contains the database definition, other files that contain data are called data sets. Root File The root file serves as a common point of entry to, and a source of information about, the database. The root file is a single-extent MPE/iX disk file; that is, the entire file occupies contiguous sectors on the disk. The person who creates the root file is the database creator and can subsequently create and initialize the database. The root file is created within the database creator's logon group and account when the Schema Processor is executed. The root file has a local file name identical to the database name. Thus, the name of the root file for the ORDERS database is ORDERS. Refer to the MPE/iX Commands Reference Manual for more information about MPE/iX account and logon groups. Data Files There is one data file for each data set of a database. The size of each record and number of records in the file are defined by the database schema, and that information is recorded in the root file. The data files are created and initialized by the DBUTIL utility. Each data file is created within the same group and account as the root file. Local file names are created by appending two characters to the local name of the root file. These two characters are assigned to the data sets according to the order defined in the schema. For example, the ORDERS database is defined with DATE-MASTER and CUSTOMER as the first two data sets. These data sets are in data files ORDERS01 and ORDERS02. (Refer to the >>CREATE command of DBUTIL in chapter 8.) Each data file is physically constructed with as many extents of contiguous disk sectors as needed to meet the capacity requirements of the file, subject to the constraints of the MPE/iX file system. Each data file contains a user label in a disk sector maintained and used by the TurboIMAGE/XL library procedures. The label contains structural pointers and counters needed for dynamic storage allocation and deallocation. Media Record Length. Media record lengths vary between data sets but are constant within each file. Each record is large enough to contain a data entry and the associated TurboIMAGE/XL pointer information. The amount of pointer information depends on the way the data set is defined. Pointer information is described in chapter 10. The maximum number of records in a data set file depends on the media record size, the available disk space, and the MPE/iX file system constraints. Blocks. The media records in a data file are physically transferred to and from the disk in groups. Each group involved in a single disk transfer is called a block. The number of records in each block is called the blocking factor. The Schema Processor determines the blocking factor during creation of the root file. Chapter 3 contains more information about block size and blocking factors. The format of blocks is given in chapter 10.