5.2. 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.

5.2.1. CREATE SHADOW

Used forCreating a shadow for the current database

Available inDSQL, 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 5.3. CREATE SHADOW Statement Parameters
ParameterDescription

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 database it is shadowing.

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.

5.2.1.1. AUTO | MANUAL Modes

When a shadow is converted to a database, it becomes unavailable. A shadow might alternatively become unavailable because someone accidentally deletes its file, or the disk space where the shadow files are stored is exhausted or is itself damaged.

  • If the AUTO mode is selected (the default value), shadowing ceases automatically, all references to it are deleted from the database header, and the database continues functioning normally.

    If the CONDITIONAL option was set, the system will attempt to create a new shadow to replace the lost one. It does not always succeed, however, and a new one may need to be created manually.

  • If the MANUAL mode attribute is set when the shadow becomes unavailable, all attempts to connect to the database and to query it will produce error messages. The database will remain inaccessible until either the shadow again becomes available, or the database administrator deletes it using the DROP SHADOW statement. MANUAL should be selected if continuous shadowing is more important than uninterrupted operation of the database.

5.2.1.2. Options for CREATE SHADOW

LENGTH

Specifies the maximum size of the primary or secondary shadow file in pages. The LENGTH value does not affect the size of the only shadow file, nor the last if it is a set. The last (or only) file will keep automatically growing as long as it is necessary.

STARTING AT

Specifies the shadow page number at which the next shadow file should start. The system will start adding new data to the next shadow file when the previous file is filled with data up to the specified page number.

☞
Tip

You can verify the sizes, names and location of the shadow files by connecting to the database using isql and running the command SHOW DATABASE;

5.2.1.3. Who Can Create a Shadow

The CREATE SHADOW statement can be executed by:

5.2.1.4. Examples Using CREATE SHADOW

  1. Creating a shadow for the current database as shadow number 1:

      |CREATE SHADOW 1 'g:\data\test.shd';

  2. Creating a multi-file shadow for the current database as shadow number 2:

      |CREATE SHADOW 2 'g:\data\test.sh1'
  |  LENGTH 8000 PAGES
  |  FILE 'g:\data\test.sh2';

See alsoSection 5.1.1, “CREATE DATABASE”, Section 5.2.2, “DROP SHADOW”

5.2.2. DROP SHADOW

Used forDeleting a shadow from the current database

Available inDSQL, ESQL

Syntax 

  |DROP SHADOW sh_num
  |  [{DELETE | PRESERVE} FILE]

Table 5.4. DROP SHADOW Statement Parameter
ParameterDescription

sh_num

Shadow number — a positive number identifying the shadow set

The DROP SHADOW statement deletes the specified shadow for the current database. When a shadow is dropped, all files related to it are deleted and shadowing to the specified sh_num ceases. The optional DELETE FILE clause makes this behaviour explicit. On the contrary, the PRESERVE FILE clause will remove the shadow from the database, but the file itself will not be deleted.

5.2.2.1. Who Can Drop a Shadow

The DROP SHADOW statement can be executed by:

5.2.2.2. Example of DROP SHADOW

Deleting shadow number 1. 

  |DROP SHADOW 1;

See alsoSection 5.2.1, “CREATE SHADOW”