Re: What is WAL used for?

> Could you tell me what is the real impact of "fsync=false" on the WAL and on the
> database in the same catastrophic scenario ?

I am not certain on this point, but I believe fsync=false messes up the
whole thing.  The nice thing about WAL is that fsync is no longer as much
of a slowdown, because PG rarely has to do random-access writes to the
disk.

Jon

>
> Thierry Missimilly
>
> Jonathan Bartlett wrote:
>
> > WAL is write-ahead logging.  Basically, before the database actually
> > performs an operation, it writes in a log what it's about to do.  Then, it
> > goes and does it.  This ensures data consistency.  Let's say that the
> > computer was powered off suddenly.  There are several points that could
> > happen:
> >
> > 1) before a write - in this case the database would be fine with or
> > without write-ahead logging.
> >
> > 2) during a write - without write-ahead logging, if the machine is powered
> > off during a write, the database has no way of knowing what remained to be
> > written, or what was being written.  WIth Postgres, this is furthere
> > broken down into two possibilities:
> >
> >   *  The power-off occurred while it was writing to the log - in this
> > case, the log is rolled back.  The database is unaffected because the data
> > was never written to the database proper.
> >
> >   *  The power-off occurred after writing to the log, while writing to
> > disk - in this case, Postgres can simply read from the log what was
> > supposed to be written, and complete the write.
> >
> > 3) after a write - again, this does not affect Postgres either with or
> > without WAL.
> >
> > In addition, WAL increases PostgreSQL's efficiency, because it can delay
> > random-access writes to disk, and just do sequential writes to the log for
> > a long time.  This reduces the amount of head-seek the dissk are doing.
> > If you store your WAL files on a different disk, you get even more speed
> > advantages.
> >
> > Jon
> >
> > On Tue, 25 Nov 2003, Relaxin wrote:
> >
> > > I'm just trying to figure out the terminology that is used on this board and
> > > wanted to know what is WAL and what roll does it play in Postgresql?
> > >
> > > Thanks
> > >
> > >
> > >
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