Re: ACID

Even Oracle is using WAL mechanism. Changes are first written to logfile before flushing the change to datafiles on disk.

For DB2, I see that its incorporated that select query from another session has to wait until User commit is 'complete'.

http://webdocs.caspur.it/ibm/db2/8.1/doc/htmlcd/en_US/index.htm?openup=admin/c0005425.htm

" The data in the log buffer is written to disk by the logger process. In the following circumstances, query processing must wait for log data to be written to disk:

• On COMMIT
• Before the corresponding data pages are written to disk, because DB2(R) uses write-ahead logging. The benefit of write-ahead logging is that when a transaction completes by executing the COMMIT statement, not all of the changed data and index pages need to be written to disk.
• Before some changes are made to metadata, most of which result from executing DDL statements
• On writing log records into the log buffer, if the log buffer is full"

On Wed, Aug 24, 2011 at 12:03 PM, <Laimutis.Nedzinskas_at_seb.lt> wrote:

>
> Implementation of durability by Oracle deserves some discussion.
>
> The term REDO is may be fine but now days there exists a better term:
> Write-ahead logging. Three words tell the essence: write to log ahead of
> modifications.
>
> Obviously for some reasons (a few of them can be named actually) oracle
> implemented the commit command using Write-after logging. One wonders what
> about rollback ? For me, Rollback seems to be fine even with write-after
> logic.
>
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> ---------------------------------------------------------------------------------
>
> Please consider the environment before printing this e-mail
> ----- Forwarded by Laimutis Nedzinskas/VBankas on 2011.08.24 12:45 -----
>
> From: Laimutis Nedzinskas/VBankas
>
> To: jonathan_at_jlcomp.demon.co.uk
>
> Cc: "Oracle L" <oracle-l_at_freelists.org>,
> oracle-l-bounce_at_freelists.org
>
> Date: 2011.08.24 12:37
>
> Subject: Re: ACID
>
>
>
>
>
> http://en.wikipedia.org/wiki/ACID
>
> There is one property which has very special comments about it:
>
> " Isolation ...
> This property of ACID is often relaxed (i.e. partly respected) because of
> the huge speed decrease this type of concurrency management implies.
> ...
> In reality, many alternatives are used due to speed concerns, but none of
> them guarantees the same reliability. "
>
> Could not be said better.
>
>
>
>
> ---------------------------------------------------------------------------------
>
> Please consider the environment before printing this e-mail
>
>
>
> From: "Jonathan Lewis" <jonathan_at_jlcomp.demon.co.uk>
>
> To: "Oracle L" <oracle-l_at_freelists.org>
>
> Date: 2011.08.23 11:54
>
> Subject: ACID
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> So who knows the meaning of ACID (not the Timothy Leary kind) ?
>
> http://tonyhasler.wordpress.com/2011/08/22/why-is-acid-important/
> http://jonathanlewis.wordpress.com/2011/08/19/redo-2/
>
>
> Regards
>
> Jonathan Lewis
> http://jonathanlewis.wordpress.com
>
>
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