Re: [HACKERS] Problem with the numbers I reported yesterday

This is 100% correct.  See my other posting describing the issues.

>
> >>>>> "ocie" == ocie  <ocie@paracel.com> writes:
>
>     > Bruce Momjian wrote:
>     >>  > > Hi, > > perhaps a stupid question: > > What will happen,
>     >> if you start PostgreSQL with -F and write a script > which is a
>     >> loop that fsyncs every 2 seconds? Does this lead to a >
>     >> database which is "almost" correct? Does this seem like a good
>     >> > compromise?
>     >>
>     >> Well, you really have to sync the data pages BEFORE sync'ing
>     >> pg_log.
>
>     > Why should this be necessary?  If the transaction is considered
>     > committed once the log has be written, and the system crashes
>     > before the data are written, then postgres can look at the data
>     > and logs when it is next started up and apply all the
>     > transactions that were logged but not committed to the data
>     > pages.
>
>     > Am I missing something?  It seems to me if you sync the data
>     > pages first, then what good is the log? (other than being able
>     > to retrace your steps)
>
> I do not think that pg_log is used like a normal 'log' device in other
> databases.  My quick look at the code looks like pg_log only has a
> list of transactions and not the actual data blocks.  Notice that
> TransRecover is commented out in backent/access/transam/transam.c.
>
> Most database log has the before images and after images of any page
> that has been modified in a transaction followed by commit/abort
> record.  This allows for only this file to have to be synced.  The
> rest of the database can float (generally checkpoints are done every
> so often to reduce recover time).  The method of recovering from a
> crash is to replay the log from the last checkpoint until the end of
> the log by applying the before/after images (as needed based on
> weather the transaction commited) to the actual database relations.
>
> I would appreciate anyone correcting any mistakes in my understanding
> of how postgres works.
>
>     > Ocie Mitchell
>
> Kent S. Gordon
> Architect
> iNetSpace Co.
> voice: (972)851-3494 fax:(972)702-0384 e-mail:kgor@inetspace.com
>


--
Bruce Momjian
maillist@candle.pha.pa.us