iX V6.5 (30391-90011)

48 Chapter2

Database Structure and Protection

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

For non-jumbo data sets, 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. For

jumbo sets and sets with B-Tree indices, the chunks or index ﬁles are created using POSIX

ﬁle format. For chunks, the local ﬁle name is appended with ".001", ".002", and so on. For

example, ORDERS01.001 and ORDERS01.002 for two chunks of ORDERS01. For index

ﬁles, the local ﬁle name of the master is appended with ".idx" (lowercase). For example,

ORDERS02.idx for the master set CUSTOMER with a B-Tree index. (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