Kbase 16541: Why use After-image? Risk losing work if you do not.
| Autor |
Progress Software Corporation - Progress |
| Acesso |
Público |
| Publicação |
10/05/1998 |
|
Why use After-image? Risk losing work if you do not.
INTRODUCTION:
=============
This Knowledgebase is to advice you to use After-imaging on your
Progress database.
WHY YOU NEED TO KNOW THIS:
===========================
After-imaging is a feature of the Progress database which copies
any updates to the database in a separate file on preferably a
separate disk. When you enable After-imaging, Progress writes
notes to the After-image (AI) file(s) that contain a record of
all changes to the database. You can use the last backup of the
database and the AI files with the roll-forward recovery
process to restore the database to the condition it was in
before you lost the database. You do not have to reprocess all
completed transactions that occurred since the last backup.
Reasons for loosing the database:
- A database file, a data extent or before-image extent is lost
because of hardware failure.
- Hardware failure corrupts a data extent or before-image extent.
- A user deleted a data extent or before-image extent.
- A user deleted accidentally data in the database.
- A backup of the database was restored over the life database while
it was active.
- The database resided in a users home directory which was removed
by the System Operator.
- A filesystem became full and damaged a data extent or
before-image extent.
- A backup script did a 'rm -r' of a temporary directory but
deleted the directory with the database in it.
Note that these are real life examples and did happen sometime in
the past.
* *
IF YOU DO NOT USE AFTER-IMAGING YOU RISK TO LOOSE THE WORK YOU
DID SINCE THE LAST BACKUP.
* *
Still a considerable percentage of our customers do not use
After-imaging on their database. You may decide to use after
imaging depending on your business needs. If you can afford to
loose a day's work (or even more depending on how often a
backup of the database is made) then after-imaging is not
necessary.
For example if the database is lost at 5 o'clock in the afternoon
you loose a day's work. It takes ALWAYS takes longer to let
the users do the same work again for that lost day. Plus you
are not sure if everything what is lost is done again, some
functions cannot be done again because of missing data.
In version 7 and later the performance impact of enabling After-
image is not as big as in version 6, in version 6 the complete
record which is updated is written to the After-image file and
in version 7 and later just the fields which have changed.
It has been reported that enabling After-image in some cases is
even faster then running without After-imaging.
PROCEDURAL APPROACH:
====================
Read all about After-imaging in the System Administration Guide,
Chapter 12.
ONLINE PROCEDURES OR UTILITIES:
===============================
The rfutil utility.
REFERENCES TO WRITTEN DOCUMENTATION:
====================================
Version 7 + 8 System Administration Guide Chapter 10 + 12.
Version 7 + 8 System Administration Reference Chapter 5.
PJO 7 Jan 1997
Progress Software Technical Support Note # 16541