Re: can the behaviour of LocalCheckPoint be changed ?

[Mikael Ronström]

Harrison Fisk wrote:

In addition to the data changes, every second counts as 32 words, thus for standard
config the LCP will run each 32768 seconds (around 9 hours).

Rgrds Mikael

> Hi,
>
> Balasubramanian Azhagappan wrote:
> > Hello Mr.Fisk,
> > Thank you for your response. I have one
> more
> > doubt regarding this.
> >
> > Harrison Fisk wrote:
> > > The LocalCheckpoint mechanism is how it
> makes
> > it
> > > possible to reuse REDO log space that
> is
> > stored on
> > > disk. In order to safely remove the
> log
> > records
> > > it needs to make sure that all
> corresponding
> > data
> > > is written to disk.
> >
> > I observed the traffic during
> the
> > Local Check point. As i was populating the
> tables
> > with 1 Million Entries, the traffic during
> each
> > LocalCheckPoint is increasing linearly. So,
> from
> > this i assumed that all the records in the
> memory
> > are written to the disk and the REDO log
> space is
> > adjusted. does corresponding rows as you
> said,
> > refers to only new records or to all ?
>
> The local checkpoint will write all of the data
> out. You can think of it basically as a rolling
> online backup that is being taken. The REDO log
> will be written out in between Local CheckPoints
> which is only the changes being made, so it
> generally is much smaller. The LCP allows the
> REDO log space to be reused.
>
> > > Keep in mind that you can set how often
> it
> > will do
> > > the LocalCheckPoint. If it is doing it
> for
> > one
> > > row, then you can bump up the setting
> > > TimeBetweenLocalCheckpoints to cause it
> to
> > wait
> > > for more data changes before doing it.
> The
> > > default is 4MB, so it shouldn't do it
> for
> > just a
> > > single row (unless you changed it).
> >
> > I am not sure whether
> > LocalCheckPoint runs when the data changes
> reaches
> > 4MB. If so, then i wonder why the
> LocalCheckPoint
> > ran even when i did not carry out any tests
> in the
> > nights, that too it ran for every 1 hour
> exactly.
> > Is it that the checkPoint interval is not
> only
> > based on data changes but also on the time
> > interval ? I have not changed the default
> value of
> > TimeBetweenLocalCheckPoint.
>
> As far as I know it is only based on Data Changes.
> Are you sure that the disk i/o you are seeing is
> based on the LCP? You can take a look in the
> cluster log and it will contain information about
> when an LCP starts and when it finishes. There
> might be a timer as well, but I imagine in
> production the data is always changing, so it
> doesn't seem like it is of consequence.
>
> If you are still worried about the amount of disk
> i/o, you can actually control how fast it is
> written out. The settings are called
> NoOfDiskPagesToDiskAfterRestartTUP and
> NoOfDiskPagesToDiskAfterRestartACC. The default
> is 3.2MB/s and 1.6MB/s and you can reduce them if
> you are worried about disk i/o during the LCP.
>
> Hope that helps clear up any confusion.
>