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IDS Forum

Re: Adding BLOB Chunks to DBSpaces

Posted By: Art Kagel
Date: Wednesday, 13 May 2009, at 2:52 p.m.

In Response To: Re: Adding BLOB Chunks to DBSpaces (DAMIAN PECNIK)

Why do you think that the physical log dbspace needs more space? Are you
looking at onstat -d? Was there some message in the message log (onstat -m)
that has given you concern?

First, the physical log is fixed size. It is initialized when the engine is
brought online during normal startup (or at the end of a successful fast
recovery after an abnormal shutdown) to the size specified in the ONCONFIG
parameter PHYSFILE. The space allocated to the physical log is treated as a
circular buffer of pages. When the physical log is 'cleared' at the end of
a checkpoint that space doesn't show as free in onstat -d, it is still
allocated to the physical log, it is just that the log begin pointer is
reset to the log tail pointer location marking it as empty.

Next, note that the physical log isn't flushed at 75% full, you
misunderstood the documentation. There are several events which will
trigger a checkpoint depending on the engine version. All versions prior to
11.50 trigger a checkpoint when the CKPTINTVL has expired or when the
physical log is 75% full. The checkpoint will eventually complete and that
will clear the physical log as described above. However, note that the log
will continue to fill if there is update activity on the server during the
checkpoint that cannot be blocked (due to sessions in critical section). If
the physical log reaches 100% full before the checkpoint completes, no harm
is done, however, some data is at risk should the server crash before the
checkpoint completes. That is one reason that the server will complain if
it thinks that the physical log is too small. These messages may be what
you are seeing.

In 11.50 and later, with non-blocking checkpoints, the physical log can
overflow into a disk file (PLOG_OVERFLOW_PATH) to increase data consistency
in case of a crash during a checkpoint. Checkpoints are triggered
autonomically based on the engine's determination of how long it will take
to recover in case of a crash compared to SLA settings you make in the
ONCONFIG file to set your maximum desirable recovery time. IBM has
determined that this autonomic feature requires that the physical log be at
least 100% of the size of the buffer pool and the engine will complain if it
is not big enough by that rubrick. You may be seeing those messages.

In the latter cases, you can increase the size of the physical log and/or
move it to a different dbspace by using onparams. The change does not take
effect until you bounce the instance. If you have to increase the size of
the physical log and it current dbspace (ROOTDBS by default) does not have
enough contiguous free space for the new log (the physical log must be a
single contiguous extent) then, yes, you will have to add another -possibly
larger - chunk to that dbspace or move the log to another dbspace that does
have enough contiguous space.

Art

Art S. Kagel
Oninit (www.oninit.com)
IIUG Board of Directors (art@iiug.org)

Disclaimer: Please keep in mind that my own opinions are my own opinions and
do not reflect on my employer, Oninit, the IIUG, nor any other organization
with which I am associated either explicitly or implicitly. Neither do
those opinions reflect those of other individuals affiliated with any entity
with which I am affiliated nor those of the entities themselves.

On Wed, May 13, 2009 at 1:44 PM, DAMIAN PECNIK <damian.pecnik@pgnmail.com>wrote:

> Ok I have been reading the documentation on plog. It says that it is
> flushed
> at 75% and you should never worry about it being full. This is one of the
> dbspaces I am trying to add a chunk to.
>
> The first circumstance under which the physical log buffer is flushed to
> disk
> is during any checkpoint. Once the checkpoint is complete, the physical log
> is
> cleared.
>
> So why does the plog need more space? Is it not getting flushed?
>
>
>
> *******************************************************************************
> Forum Note: Use "Reply" to post a response in the discussion forum.
>
>

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