Dump Files

Shaker (c21vc@mail.delcoelect.com) wrote:
: 	I am trying to understand how dump files work, by reading
: Mike Leo's INGRES World '94 paper 'Safe and Recoverable OpenIngres
: Database Backup'.

I don't know that paper, but I hope, I can help you anyway.

:  . Between 12.02 and 12.03, an index on table FOO is backed up

:  . At 12.06, a transaction arrives which deletes row X from table FOO
:    (and from table FOO's index - assuming a single index). 

:  . At 12.12, table FOO is backed up; the backup will not contain row X;
:    however, the index (which was backed up before the delete transaction)
:    will contain row X

: Mike says that rollforwarddb will use the dump files to fix this
: situation and 'assure row X is in the table FOO, and in the index on FOO,
: thus preserving the index integrity.' (Page 6 of the paper.) Shouldn't
: the dump file be used to remove row X from the index instead of adding
: row X to the table ? Or am I missing something ?

The purpose of the dump files is to store any information needed to
perform a rollback of all transactions which changed the database while
it was backed up.  In case of a rollforwarddb this rollback takes place
and therefore rebuilds the row X in table FOO.  I.e. after applying the
dump files, the database is in the consistent state of that point in time,
when the backup was started.

If your database is journalled (and table FOO as well), rollforwarddb will
apply the journal files to re-execute all transactions and thus it will
remove row X again.  This is the default behavior, you might suppress
applying the journal files with a "rollforwarddb -j".

: In the same item, Mike uses the term 'before-image of a transaction'. What
: does this refer to ?

To be more precise, you have 'before-images of the pages that are changed
by a transaction'.  These before-images are always stored in the log file
(to enable the rollback of any open transaction), and while an online-
checkpoint is running, they are stored in the dump file, too (to perform
a consistent rollforwarddb, as I describe above).

: Thanks.

: Shaker

'welcome,
	Christopher
-- 
____________________________________________________________________________
Christopher Etz            Kopernikusstr. 28          D-65929 Frankfurt/Main
cetz@cetz.rhein-main.de    Tel.: +49 69 318091        Telefax: +49 69 318091




> 
> 
> 	I am trying to understand how dump files work, by reading
> Mike Leo's INGRES World '94 paper 'Safe and Recoverable OpenIngres
> Database Backup'. In Item 8 of 'How Checkpoints Work', he sets up the
> following scenario of an online checkpoint:
> 
>  . Between 12.02 and 12.03, an index on table FOO is backed up
> 
>  . At 12.06, a transaction arrives which deletes row X from table FOO
>    (and from table FOO's index - assuming a single index). 
> 
>  . At 12.12, table FOO is backed up; the backup will not contain row X;
>    however, the index (which was backed up before the delete transaction)
>    will contain row X
> 
> Mike says that rollforwarddb will use the dump files to fix this
> situation and 'assure row X is in the table FOO, and in the index on FOO,
> thus preserving the index integrity.' (Page 6 of the paper.) Shouldn't
> the dump file be used to remove row X from the index instead of adding
> row X to the table ? Or am I missing something ?
> 
> In the same item, Mike uses the term 'before-image of a transaction'. What
> does this refer to ?
> 
> Thanks.
> 
> Shaker
> 
> 
Shaker,

Mike is right.  The dump file is used to restore the database to the
state it was in at the START of the checkpoint.  The deletion of row
X is achieved by applying the journals which are the changes since
then.  The "before-image of a transaction" is the state of the data
before the changes occur.  On a record-by-record basis, for an update
it is the "old record", for an insert it is "no record", for a delete
it is "the record".

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