On 4/18/2017 2:25 AM, Vernooij, Kees - KLM , ITOPT1 wrote:
As I said, I remember reading this a long time ago, I don't know the details
anymore, whether the source was reliable and whether it is still working this
way. Only a DB2 internal expert should be able to tell.
...
On 12 April 2017 at 10:16, Tom Marchant <
0000000a2a8c2020-dmarc-requ...@listserv.ua.edu> wrote:
[DB2]
So, if it thinks it would be faster to read the record from DASD than
for
MVS to page in the buffer page(s) containing the record, it will read
the
record into different pages that have not been paged out?
While the DB2 buffer manager code does collect statistics on the
frequency of buffer pages being latched that are currently paged out
(using TPROT), I don't remember seeing any logic to reread a buffer page
vs waiting for a demand page-in to occur in the code. And given that
the current copy of a page may be the one in the buffer pool, rather
than on the copy on DASD, and that multiple threads can have shared
ownership of a buffer pool page, I do not believe this processing exist
(at least for the last 15 years) in DB2.
30 years ago this type of logic, to reread from DASD rather than demand
page-in a buffer page, *might* have made sense, when the I/O bandwidth
to the paging subsystem was limited based on the number of page datasets
and a demand page rate of 30-100 pages a second was common to see. But
the world has changed and it would not make sense to do it today.
Regards,
Greg
----------------------------------------------------------------------
For IBM-MAIN subscribe / signoff / archive access instructions,
send email to lists...@listserv.ua.edu with the message: INFO IBM-MAIN
