Re: Exact meaning of TCH in X$BH

It would have been easier to guess if spelt "THC"... ;)

Happy New Year!

-- 
Martin Klier // Performing Databases GmbH 
Managing Partner // Senior DB Consultant 
Oracle ACE Director 
Member of Symposium42 

martin.klier_at_performing-db.com // https://www.performing-databases.com 

> Von: "l flatz" <l.flatz_at_bluewin.ch>
> An: "Stefan Köhler" <contact_at_soocs.de>, "Oracle-L Freelists"
> <oracle-l_at_freelists.org>
> Gesendet: Freitag, 30. Dezember 2022 13:47:35
> Betreff: Re: Exact meaning of TCH in X$BH

> Thanks Stefan and happy new year!

> Am 30.12.2022 um 12:27 schrieb Stefan Koehler:
> > Hello Lothar,


>> the TCH can be way higher ... but ... quoting from Jonathan's book "Oracle Core:


> > Essential Internals":
> > -------8<-----------


>> Oracle Corp. added a counter (and a timestamp—called tim) to the buffer header,

>> and every time someone visits the buffer, they increment the touch count and


>> update the timestamp—provided that at least 3 seconds has passed since the last


> > update
> > ...


>> There is a commonly held theory that you can identify which block is causing

>> latch contention on a given cache buffers chains latch by checking for very

>> high touch counts (TCH) on all the buffers covered by that latch. Unfortunately

>> this is not a very sound method. A buffer that is visited an average of once

>> per second for half an hour will have a touch count around 600; a buffer that

>> has been visited 10 million times in the last 5 minutes will have a touch count


>> of around 100. The touch count can give you a clue, but it is not the final


> > answer.
> > ...
> > -------8<-----------

> > Hope this explains the behavior you are seeing.

> > Best Regards
> > Stefan Koehler

> > Independent Oracle performance consultant and researcher
> > Website: http://www.soocs.de
> > Twitter: _at_OracleSK

> >> Lothar Flatz <l.flatz_at_bluewin.ch> hat am 30.12.2022 11:58 CET geschrieben:


> >> Hi guys,

>>> the official definition is : "X$BH.TCH is a touch count for the buffer. A high
> >> value for X$BH.TCH indicates a hot block."
>>> (Source:
> >> https://docs.oracle.com/en/database/oracle/oracle-database/19/tgdba/instance-tuning-using-performance-views.html#GUID-64F78817-8B4C-4392-B518-CA31CF728B69).

>>> But the biggest value for tch I have ever seen is 255 (unsigned seven bit
>>> integer) . Therefore I think it is not exactly a counter but rather reflects a
> >> range.
> >> Can somebody give a detailed explanation? Does the number wrap around? When?

> >> Thanks

> >> Lothar
> > --
> > http://www.freelists.org/webpage/oracle-l



> --
> http://www.freelists.org/webpage/oracle-l
--
http://www.freelists.org/webpage/oracle-l