IDS Forum

Re: onstat -g ckp - block x wait

Posted By: Cesar Inacio Martins
Date: Tuesday, 31 August 2010, at 12:45 p.m.

In Response To: Re: onstat -g ckp - block x wait (JACQUES RENAUT)

Nice!

Thanks for this more detailed explanation.
On the manual is missing an information explained like this...

Just for curiosity.
One of cases where I see longs waits into the "wait time" is mainly because of the high RTO where practically one or no checkpoints are executed during the day, (mostly is forced when the archive is started).

Regards
Cesar

--- Em ter, 31/8/10, JACQUES RENAUT <jrenaut@us.ibm.com> escreveu:

De: JACQUES RENAUT <jrenaut@us.ibm.com>
Assunto: Re: onstat -g ckp - block x wait [21086]
Para: ids@iiug.org
Data: Terça-feira, 31 de Agosto de 2010, 10:13

Original post:
Hi Jacques,
Thanks for your answer...
Help, but I still have the doubt... for how much time really/effective happen
a blocking checkpoint ? is the block time + wait time?
As you says, isn't every time the wait time means blocking or non-blocking ...
how to know?

Response:
Actually, the block time and wait time are both blocking, but the causes for
them are completely different. If you are getting block time from the onstat
-g ckp output, then that means you are running out of resources before the
buffer pool can be flushed in a non-blocking manner, and the checkpoint has to
switch from a non-blocking buffer pool flush, to an old style checkpoint where
the users need to be blocked from entering critical sections (or basically
doing any writes/updates) while the remainder of the buffer pool is then
finished up being flushed.

Wait time, on the other hand, is also blocking, but it is blocking that is
happening during the blocking portion of the new non-blocking flushing of the
buffer pool checkpoint code. Basically, prior to starting the buffer pool
flushing, there is still a blocking portion of the checkpoint code where users
are still prevented from entering critical sections. Again, the only
non-blocking portion of the new checkpoint code is the period while the buffer
pool is being flushed to disk. Now, generally speaking, that should be the
largest chunk of time for a checkpoint, but it isn't the only thing that a
checkpoint needs to do, so the new non-blocking checkpoints, do not mean
people never wait. The non-blocking portion is only while the buffer pool
flushes.

If you are seeing large "wait time" column output, then that means the server
is having to spend a lot of time to complete what it needs to get done during
the blocking portion of the new checkpoint code. It can be caused by a number
of things, so without more information it would be hard to say why you are
seeing this.

But again to answer your question, both block time and wait time are blocking.
However, if you are just looking for total amount of time that is spent
blocked, you would need to add up the block time column along with the ckpt
time column, and finally the Long time column. The sum of those 3 columns
should be pretty close to the amount of time during that checkpoint where
users would have been prevented from doing write type activity, but all 3 of
those columns have different reasons for what would be causing them.

The "Wait time" column is the avg length of time any waiter had to wait during
the blocking portion of the checkpoint. It's calculated by adding up all the
time that any thread that had to wait, and dividing it by the number of
threads that had to wait, to get the avg length of time spent waiting.

Jacques Renaut
IBM Informix Advanced Support
APD team

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Messages In This Thread

• onstat -g ckp - block x wait
  Cesar Inacio Martins -- Friday, 27 August 2010, at 3:03 p.m.
  • Re: onstat -g ckp - block x wait
    JACQUES RENAUT -- Friday, 27 August 2010, at 3:21 p.m.
    • Re: onstat -g ckp - block x wait
      Cesar Inacio Martins -- Tuesday, 31 August 2010, at 6:53 a.m.
      • Re: onstat -g ckp - block x wait
        JACQUES RENAUT -- Tuesday, 31 August 2010, at 9:13 a.m.
        • Re: onstat -g ckp - block x wait
          Cesar Inacio Martins -- Tuesday, 31 August 2010, at 12:45 p.m.

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