SHADOW

A shadow is an exact, page-by-page copy of a database.Once a shadow is created, all changes made in the database are immediately reflected in the shadow.If the primary database file becomes unavailable for some reason, the DBMS will switch to the shadow.

This section describes how to create and delete shadow files.

`CREATE SHADOW`

Creates a shadow file for the current database

Available in

DSQL, ESQL

Syntax

CREATE SHADOW <sh_num> [{AUTO | MANUAL}] [CONDITIONAL]
  'filepath' [LENGTH [=] num [PAGE[S]]]
  [<secondary_file> ...]

<secondary_file> ::=
  FILE 'filepath'
  [STARTING [AT [PAGE]] pagenum]
  [LENGTH [=] num [PAGE[S]]]

Table 1. `CREATE SHADOW` Statement Parameters
Parameter	Description
sh_num	Shadow number — a positive number identifying the shadow set
filepath	The name of the shadow file and the path to it, in accord with the rules of the operating system
num	Maximum shadow size, in pages
secondary_file	Secondary file specification
page_num	The number of the page at which the secondary shadow file should start

The CREATE SHADOW statement creates a new shadow.The shadow starts duplicating the database right at the moment it is created.It is not possible for a user to connect to a shadow.

Like a database, a shadow may be multi-file.The number and size of a shadow’s files are not related to the number and size of the files of the shadowed database.

The page size for shadow files is set to be equal to the database page size and cannot be changed.

If a calamity occurs involving the original database, the system converts the shadow to a copy of the database and switches to it.The shadow is then unavailable.What happens next depends on the MODE option.