Hi Laimutis, 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@xxxxxx> 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. > > > > > > > > > > > --------------------------------------------------------------------------------- > > 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@xxxxxxxxxxxxxxxxxx > > Cc: "Oracle L" <oracle-l@xxxxxxxxxxxxx>, > oracle-l-bounce@xxxxxxxxxxxxx > > 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@xxxxxxxxxxxxxxxxxx> > > To: "Oracle L" <oracle-l@xxxxxxxxxxxxx> > > Date: 2011.08.23 11:54 > > Subject: ACID > > > > > > > > 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 > > > -- > //www.freelists.org/webpage/oracle-l > > > > > > > -- > //www.freelists.org/webpage/oracle-l > > >