RE: question on initrans

Pap, I think you mean ASSM which is automatic segment space management and not ASM which has to do with database file space allocation. With or without ASSM being in use Oracle automatically manages ITL allocation. If a session attempts to update a row and the block the row is in does not have a free ITL Oracle automatically attempts to allocate an ITL in the block. If space for the ITL is not available then the session has to wait for one to become available resulting in an ITL wait .

When you know that multiple DML processes will be updating rows in the same block then pre-allocating ITL’s to support these transactions is a good idea, but the number chosen should be realistic for the specific object in question.

From: oracle-l-bounce_at_freelists.org <oracle-l-bounce_at_freelists.org> On Behalf Of Pap Sent: Tuesday, May 11, 2021 11:41 PM
To: K Gopalakrishnan <kaygopal_at_gmail.com> Cc: Oracle L <oracle-l_at_freelists.org> Subject: Re: question on initrans

Thank you so much for this detailed explanation.

If I get it correctly, it's the ASM tablespace(which we do have) manages the dynamic increase and decrease of ITL or initrans based on concurrency so we don't' need to set them explicitly. Also it should be looked into only if by we see the ITL related waits. Correct if my understanding is correct. As you mentioned in case of high concurrency we may need to increase pctfree to higher value, will that not automatically managed by ASM and also in such cases do we see any other waits or same ITL waits will be seen in AWR?

Is freelists also managed dynamically as above and no need manually to be set to higher on a ASM? (Note-Actually we are encountering buffer busy waits during high activity period and the object it's pointing to is table followed by index.)

Regards
Pap

On Wed, 12 May 2021, 8:20 am K Gopalakrishnan, <kaygopal_at_gmail.com<mailto:kaygopal_at_gmail.com>> wrote: Pap-

The parameter INITRANS - defines number of ITL (Interested Transaction Lists) slots _initially_ created during new blocks allocation to the segment. Any transaction that is interested in conducting DMLs on that block, should fill in some details about that transaction. This includes setting the Transaction ID, Undo Block Address, Lock Byte , Cleanout System Change Number and Free Space Credit. The row level locking in Oracle is implemented with the help of these ITL information as well.

In other words, treat ITL like your parking space in the office space. Anyone entering the office must park their car in the parking space allotted. You can park your car in the allotted space or any free space in that building. Alloted parking spaces are fixed ITLs during the creation of the block. If allocated parking space is full, you park in the free space in the building. Once you use this free space, this space will never return data storage, only for ITLs. This defaults to 2 per table and 3 per index IIRC, but DBA_TABLES still show the default as 1 per table and 2 per index. You can quickly validate this by simple block dumps.

Note that the ITLs can be created when there is enough space (~24 bytes per ITL) in the variable header space in the data block. So when there is a demand for additional transaction slots, oracle automatically creates ITL slots subject to free space in the block. If there is no space in the block , you might see contention for ITLs and you will see ITL waits in the session_wait views.

There are some internal validations in oracle to limit the variable part of the block header that can not exceed 50% of the block size. i.e you can not create 50 ITLs if the block size is 2K. (50 ITLs require 50x24=1200+ bytes which is clearly over 50% of the block size). There are many other checks similar to these as well to stop the abuse :)

Also the requirement of such high ITL depends on the number of active rows per block. If the block itself has less than 50 rows, you would neer need 50 ITLs for that block. Similar to parking slot analogy, you would not need more parking spots than the number of residents. Readers (aka visitors) do not need any ITL entries to record their transactions.

You do not need to manually set the ITLs, as ITLs will be created on the need basis. If there is an issue with the ITLs , you will clearly see this in the session_waits. Also segment_stats has specific metrics to show the objects with ITL contention. If you are not seeing any of this in the DB, this recommendation from oracle is totally irrelevant. So 50 ITLs (per table & 100 ITLs on their indexes) seems such a waste of block space.

There are much better mechanisms to handle this. If you suspect such a high rate of concurrency, please set the PCTFREE to a higher number (say 15%) so that additional ITLs can be created when required. The space reserved with PCTFREE can not be used for INSERTS. They are used for ITL expansion and updates.
-Gopal

On Tue, May 11, 2021 at 11:47 AM Pap <oracle.developer35_at_gmail.com<mailto:oracle.developer35_at_gmail.com>> wrote: This database was recently moved from HP to exadata X7 and the DB version also migrated from 11.2.0.4 to 19.9.0.0.0. It's an OLTP database. In the past we saw some contention and got a recommendation from Oracle to set INITRANS value of certain objects(both table and indexes) to as high as 50. And now the team is thinking to set the initrans value to ~10 for almost all the objects(~around ~200+ objects). Want to understand , what is the negative side of setting it to this non default value considering this is an OLTP database exposed to many concurrent DML/query and we encounter "buffer busy waits" many times?

Regards
Pap

--

http://www.freelists.org/webpage/oracle-l Received on Wed May 12 2021 - 16:38:54 CEST