GBPCHKPT setting

DANIEL CYR

GBPCHKPT setting
All,

I was wondering if there is anything written about the setting of the
coupling facility checkpoint frequency? What I do find is very little
and to tell you the truth of NO value. Currently we have BMC's Pool
Advisor and it mentions the the GBPCHKPT should be set between 5 and 15.
Currently the coupling facility was brought up when DB2 v4.1 was here
and the default setting (4 minutes) is still in effect.

One question, when the GBPCHKPT threshold is reached (4 minutes) is
everything cast out? What happens when the threshold is exceeded?

If anyone has any material on how and why to sent CLASST, RATIO,
GBPCHKPT please share it with the list.

DB2 v7.1
8 member sysplex
z/os 1.2.0

TIA,

Dan Cyr

DB2 DBA
Stegmaier Building
United States Postal Service, WBIBSSC
7 North Wilkes-Barre Blvd, Suite 5
Wilkes-Barre, PA 18702-9200
office: 570-830-5841
e-mail: [login to unmask email]

Susan Lawson

Re: GBPCHKPT setting
(in response to DANIEL CYR)
Daniel,

First, remember that the CLASST, GBPOOLT and GBPCHKPT are defined for each
group bufferpool. So you can begin with the standard - it depends, when it
comes to the best settings. It depends on what time of usage is against the
data in the group bufferpool (will depend on the separation of your objects
in your virtual pools). So if you have a high rereference of pages, then you
do not want them to be constantly castout (which is the process these
thresholds control - similar to the DWQT and VDWQT control of writes in
virtual pools). The GBP Checkpoint acts somewhat similar to the checkpoint
on a single subsystem, except it occurs for an individual GBP - monitored by
the structure owner. When it is reached, DB2 begins to castout pages from
the changed class queues to DASD (performed by the castout owner). The best
set interval is one that achieves balance between the performance impact of
frequent group bufferpool checkpoints and the impact to recovery. The lower
the checkpoint interval the higher the resource consumption due to more
frequent checkpoints. The higher the checkpoint interval the slower the
recovery from a group bufferpool failure due the extended period of time
between checkpoints. However, if you are duplexing the recovery time may
not be much of an issue. Also, you may want to double check your
settings...the default is 8 minutes..so some where along the line yours must
have been changed for some reason. Also, just like with control writes on a
virtual pool, it is better to control the castout of changed with the CLASST
(percentage of changed pages for a particular class quere) and GBPOOLT
(percent of changed pages in GBP) thresholds.

The ratio is a setting on the group bufferpool that establishes the number
of directory entries to the number of data entries in the GBP. If we do not
have enough directory entries (entry for each page read on any DB2 member -
only one page registered regardless of number of members with interest),
then when a new page needs to be registered a directory will be reclaimed in
order to register a new page. This will then cause process requiring that
page to have to go to disk to reread and register the page. Depending on the
number of time this occurs it can add up to significant overhead. Use
the -DISPLAY GROUPBUFFERPOOL command to determine how many times this
occurs. IFCID 0255 can also be used to determine the number occurrences of
a buffer refresh caused by cross-invalidation of a data page in the GBP to
help you get an idea of the amount of read/updates occurring among the
members. The default is 5...which you could think of as 5 pages read for
every one 1 updated...not best in most cases.

If you need more information please feel free to email me directly.

Regards,
Susan Lawson

********************************************
* Susan Lawson
*
* DB2 for OS/390 Certified Solutions Expert *
* IBM DB2 and S/390 Gold Consultant *
* YL&A, Inc
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* www.ylassoc.com
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-----Original Message-----
From: DB2 Data Base Discussion List [mailto:[login to unmask email]On
Behalf Of Cyr, Daniel P - Wilkes Barre, PA
Sent: Sunday, January 05, 2003 12:26 PM
To: [login to unmask email]
Subject: GBPCHKPT setting


All,

I was wondering if there is anything written about the setting of the
coupling facility checkpoint frequency? What I do find is very little and
to tell you the truth of NO value. Currently we have BMC's Pool Advisor and
it mentions the the GBPCHKPT should be set between 5 and 15. Currently the
coupling facility was brought up when DB2 v4.1 was here and the default
setting (4 minutes) is still in effect.

One question, when the GBPCHKPT threshold is reached (4 minutes) is
everything cast out? What happens when the threshold is exceeded?

If anyone has any material on how and why to sent CLASST, RATIO, GBPCHKPT
please share it with the list.