Kbase 5967: How does online probkup work; Impact with .ai and .bi files
| Autor |
Progress Software Corporation - Progress |
| Acesso |
Público |
| Publicação |
05/02/2002 |
|
How does online PROBKUP work; Impact with .ai and .bi files
910429-CHG01The first thing that Progress does when you make an online
backup of your database is make a backup copy of the .bi file
as it exists at the start of the backup. While this is
going on, no transactions can cause writes to the bi file.
Consequently, users are 'frozen' for as long as this takes.
Having backed up the .bi file, the next stage is to
backup the database starting at block 0 through to the end,
which in this context refers only to the current high water
mark. It is important to note that performing a full
online backup does not include formatting of a multi-volume
database. You must perform a restore into a void structure
otherwise you will end up with a single-volume database.
During the backup process Progress checks the shared
memory dirty chain (blocks with modifications pending).
Any blocks found on this chain are backed up immediately
so that the modification of the block can proceed.
Subsequently, the block is skipped when it is reached in
the database.
Backup proceeds in this way until complete. In order to
minimize the impact on performance, the priority of the
backup process can be increased on UNIX systems using
the 'nice' command.
It should be noted that the backup created in this way is
a snapshot of the database at a moment in time. As a result,
it is like backing up a database on which the plug has just
been pulled. This means that rollback recovery will take
place and a maximum of one transaction per user may be lost.
Since the ONLINE option only works for multi-user databases,
DO NOT try to use the online option for PROBKUP if you are:
- Running a system without shared memory, such as DOS
(including NLM) or BTOS
- Running your databases in single user mode, even on
UNIX, OS/2, or VMS.
In version 6 and earlier, you MUST DISABLE after-imaging if you are
performing an online backup. Since disabling after
imaging (rfutil AIMAGE END) requires the server to be
taken down, you effectively CANNOT use online backups
for a database where after-imaging is used to capture
recoverable transactions.
In version 7 and later, online backups can be used without disabling
after imaging if the ai file is multi-volume. If the ai file is single
volume, then the database would have to be brought down for the backup.
Progress Software Technical Support Note # 5967